Environment international volume 73 issue 2014

his paper discusses possible health implications related to dust particles released during the manufacture of sheeps woolbased nonwoven insulation material. Such insulation may replace traditional synthetic insulation products used in roofs, wall cavities, etc. A review of the literature concerning organic dusts in general and sheeps woolfiber summarizes dust exposure patterns, toxicological pathways and the hazards imposed by inhalation and explosion risk. This paper highlights a need for more research in order to refrain from overgeneralizing potential pulmonary and carcinogenic risks across the industries. Variables existing between industries such as the use of different wool types, processes, and additives are shown to have varying health effects. Within thefinal section of the paper, the health issues raised are compared with those that have been extensively documented for the rock and glass wool industries

Review Assessment of health implications related to processing and use of natural wool insulation products E. Mansour ⁎ , C. Loxton, R.M. Elias, G.A. Ormondroyd The BioComposites Centre, Bangor University, Deiniol Road, Bangor, Gwynedd, LL57 2UW, United Kingdom abstractarticle info Article history: Received 7 March 2014 Accepted 5 August 2014 Available online 18 September 2014 Keywords: Wool Natural fiber Insulation Dust Health Exposure This paper discusses possible health implications related to dust particles released during the manufacture of sheep's wool-based non-woven insulation material. Such insulation may replace traditional synthetic insulation products used in roofs, wall cavi ties, etc. A review of the literature concerning organic dusts in general and sheep's wool fiber summarizes dust exposure patterns, toxicological pathways and the hazards imposed by inha- lation and explosion risk. This paper highlights a need for more research in order to refrain from overgeneralizing potential pulmonary and carcinogenic risks across the industries. Variables existing between industries such as the use of different wool types, processes, and additives are shown to have varying health effects. Within the final section of the paper, the health issues rais ed are compared with those that have been extensiv ely documented for the rock and glass wool industries. © 2014 Elsevier Ltd. All rights reserved. Contents 1. Introduction 403 1.1. Natural fiberuseandconcerns 403 1.2. Sheep'swool-basedinsulationprocessingandinstallationoverview 404 1.2.1. Scouringofrawwool 404 1.2.2. Productionofinsulationproductsfromsheep'swool 404 1.2.3. Installationoverview 405 2. Potentialhealthrisks 405 2.1. Organicdust 405 2.1.1. Exposuretoorganicdust 405 2.1.2. Hazardsassociatedwithorganicdust 406 2.2. Wool fiberanddust 406 2.2.1. Exposure 406 2.2.2. Hazards 407 2.2.3. Toxicology 407 2.3. Comparison with synthetic fiber 408 2.3.1. Exposure 408 2.3.2. Hazards 408 3. Conclusion 408 Acknowledgments 410 References 410 Environment International 73 (2014) 402–412 Abbreviations: WHO, World Health Organisation; BOHS, British Occupational Hygiene Society; D ae , Aerodynamic diameter; D, Particle diameter; ρ,particledensity;ρ o ,Densityof water; TLV, Threshold Limit Values; HSE, Health and Safety Executive; FEV 1 , Forced expiratory volume in 1 second; TNF, Tumor necrosis factor; OSHA, Occupational Safety and Health Administration; HSPP, Health and Safety Partnership Program; FVC, Forced vital capacity. ⁎ Corresponding author. Tel.: +44 1248 370588; fax: +44 1248 370594. E-mail addresses: e.mansour@bangor.ac.uk (E. Mansour), c.loxton@bangor.ac.uk (C. Loxton), r.m.elias@bangor.ac.uk (R.M. Elias), g.ormondroyd@bangor.ac.uk (G.A. Ormondroyd). http://dx.doi.org/10.1016/j.envint.2014.08.004 0160-4120/© 2014 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Environment International journal homepage: www.elsevier.com/locate/envint 1. Introduction Effects specifically concerning exposure to inhaled particles, starting with coalmine dust, have been a subject of concern since 1960, when the first Inhaled Particles Symposium was held by the British Occupa- tional Hygiene Society (BOHS). The focus started with establis hing standards for controlling coalmine dust, and expanded in the 1980s to include fibers such as asbestos. This on-going international symposium is still expanding its scope to include nano-particles and toxicological mechanisms (British Occupational Hygiene Society, 2013). Lung condi- tions have been a serious issue worldwide, and are estimated to be the cause of 1 in 10 of all deaths, costing more than €380 billion annually in Europe at 2011 values. These data were collated by the World Health Organization (WHO) and the European Centre for Disease Prevention and Control, and is sum mariz ed in the European Lung Whit e book (European Respiratory Society, 2013). Various agents found in the workplace are shown to be responsible for: about 15% in men and 5% in women of all respiratory cancers; 17% of all adult asthma cases; 15 to 20% of chronic obstructive pulmonary disease cases; and 10% of interstitial lung disease cases. The research concludes that assessing occupational exposure risks and diagnosis requires a detailed historical study. 1.1. Natural fiber use and concerns The thermal insulation sector for the domestic building sector (e.g. in lofts, cavity walls, etc.) has been an important one since the com- pulsory push to insulate buildings and meet energy targets have been set over 30 years ago (Papadopoulos, 2005). The sector is dominated by synthetic inorganic fibrous insulation and organic foams. Although thermal properties of insulation materials have not appreciably im- proved over the past decade, there is a growing interest for additional functionalities by some end users: moisture buffering, mechanical durability, breathability, sustainability, etc. The popularity of the latter properties has increased the interest in natural materials. Fibers used for non-woven insulation products can be categorized as natural or industrial as per Fig. 1. Although it can be argued that indus- trially produced fibers are ‘natural’ for the purpose of this paper the term ‘natural fibers’ refer to all naturally polymerized fibers of plant and animal origin; and the term synthetic fiber is used to mean fiber- glass, rock or slag wool. The use of natural fibers, including sheep's wool, is an area of increasing interest, and opportunities are being develop ed in new markets (Karus and Kaup, 2002; Khedari et al., 2004). Historically, natural fibers have been used extensively by the textile industries. Nowadays, accumulating research has highlighted their attractive prop- erties and benefits: efficient thermal resistivity (Fan et al., 2008), good structural strength (Feughelman, 1997; Wambua et al., 2003), moisture buffering capacity (Watt, 1960), and the uptake of certain gasses (Curling et al., 2012 ). Natural fibers as raw materials for insulation have attracted atten- tion because of their historical use, availability, and sustainability. Their availability is evident from their large usage in textile and other industries, such as carpet manufacturing. New legislation further com- plements the drive to natural fiber use (UK Government, 2013)asit pushes to reduce energy usage and sequester carbon emissions (Barbier, 2010). This reflects on the natural fiber composites market, which is expected to grow to US $531.3 million in 2016 (Research and Markets, 2011). In addition, the econ omics of some natural fibers is structured; for example, although the demand for sheep's wool fluctu- ates (Boutonnet, 1999)—leading to a reduction in price and frustration among farmers—prices are regulated by the British Wool Marketing Board (British Wool Marketing Board, 2005), Australian Wool Exchange (Australian Wool Exchange, 2013), and oth er authorities in various countries. Therefore, it can be concluded that processing of such fibers into insulation products is encouraging from a performance, environ- mental, and a wider economic view. There are many indications about the environmental benefits and actual health benefits associated with the presence of natural insulation material in buildings (Dewick and Miozzo, 2002; Korjenic et al., 2011): moisture buffering properties decrease occupant discomfor t; the natural materials' characteristic odors are reported to positively influ- ence the human psyche; and indeed the anecdotal evidence of the manufacturers appears to show that the insulation is easier and less dan- gerous to the health of the installer and the end user (Black Mountain Insulation, 2012; Thermafleece, 2013). Raw materials Natural fibres Vegetable/cell- ulose based Seed fibre CoƩon Bast fibre Flax, hemp, kenaf Fruit fibre Coire Wood fibre Wood wool, paper Animal/proƟen based Hair Sheep's wool Industrialy produced fibres Organic Starch PolylacƟc acid Crude oil Polystyrene Inorganic Diabas, basalt Rock wool Quartz Mineral wool Fig. 1. Categorization of fibers used for non-woven insulation production (Müssig and Graupner, 2010). 403E. Mansour et al. / Environment International 73 (2014) 402–412 The purpose of this review is to assess the scientific findings related to the health and safety aspects of manufacturing insulation material based on natural fibers, specifically sheep's wool fiber—hereon referred to simply as ‘wool’. A comparison of these fin dings with th ose of synthetic fiber-based insulation shows that health risks are more exten- sively established for the latter. Most studies referenced in this review do not cover the natural fiber insulation industry, but relate to the tex- tile industry. This highlights the lack of at tention to pr imary sources of information for these emerging specialized industrial sectors which can vary significantly from ‘traditional’ textile industries in the way they process the raw fiber. When studies are categorized according to exposure and hazards during manufacture, installation, use and retrofit/removal (Table 1 and Fig. 2), it is apparent that hazards during manufacture have been the focal point, where research concerning carcinogenic effects account for well over half of the studies for synthetic fiber. Exposure levels seem to be studied to a lesser extent, especially when it comes to installation, use, retrofit or removal. Interestingly, there are no exposure studies for wool fiber except during manufacturing; possibly, this is due to these fibers having a much smaller and more recent market share when compared to synthetic fiber. 1.2. Sheep's wool-based insulation processing and installation overview Sheep's wool stands out from other fibers. It is the only protein- based one utilized by the insulation industry. Its structure is known to be composed of cortical cells, surrounded by cuticular scales as shown in Fig. 3 (Bonès and Sikorski, 1967). It differs vastly from synth etic fibers, which tend to be a random mix ture of silica and other metal oxides (Andrew Dunster, 2007). 1.2.1. Scouring of raw wool The scouring process essentially cleans the wool from inorganic and organic contaminants: grease, dirt, vegetable matter and manure are removed along with preventable health risks (see Section 2.2.2)and processing difficulties (large entanglements and debris that can disrupt production or damage machinery) at further manufacturing stages. Scouring starts with mixing different wool types and grades to ob- tain a desired blend. The types of wool mixed together determine the blend's grade, which in turn is dependent on the manufacturer and end use application. Textile processors generally opt for the finer grades of wool that are characterized by smaller fiber diameters (fineness), a homogenous color, and low crimp factor (natural waviness or bend in the fiber). Carpet manufacturers may compromise on the fineness of the blend to a certain extent, but generally remain interested in color. Insulation manufacturers on the other hand opt for the coarser, lower grade blends with complete disregard for color. The choice of blend and grade therefore determines physical characteristics of the wool fiber such as density and fragmentation patterns. The blend is passed through openers that are composed of a relative- ly large teethed rotating roller that fragments large tufts of wool. Fol- lowing that it is cross laid and re-blended to ensure proper mixing and washing further down the line. Before washing, the wool is picked up by inclined lattices, allowing the dust to be collected at the bottom. After further mixing, the wool is washed. There are several methods for washing; solvent based systems are usually unpopular, unlike aque- ous systems (Stewart, 1988). Aqueous treatment involves several steps of mixing with detergents and rinsing. Depending on the wool blend, process parameters such as washing temperatures are adjusted. After the pH is controlled and other treatments (such as brightening) are ap- plied, a sample of wool is tested for color and bound moisture content. If satisfactory, the batch is dried, checked for the presence of metal and non-conformities, and re lieved of large clumps and entanglements. The effluent water is usually treated on site as it is the most expensive process. Further optional treatment can be applied using Andar and similar systems that apply a partial vacuum to remove dust. After further cross-layering and blending through inclined lattices (which again removes remaining dust) and some final tests, the wool is baled as a final product. Wax, dirt, and water soluble material vary in amount depending on the breed, age, and location of the sheep, but is typically between 30 to 47% (Johnson and Russell, 2008). 1.2.2. Production of insulation products from sheep's wool Non-wo ven insulation is composed of low grade scoured wool mixed with an appropriate proportion of a binder, usually a low melt synthetic fiber such as polyester or a wet adhesive; needle felting is an- other option that removes the need for a binder, but the thicker the final product the lower structural integrity it will have. Treatments including flame retardants and pesticides may be applied at the mixing stage to ensure proper distribution throughout the thickness of the final prod- uct, or may be sprayed on the surface of the final product at later stages. The wool and binder mixture first passes through an opener that partly homogenizes the mix in addition to loosening compact fiber bundles. Next, it is scanned for any metal content (as this can damage the machinery) and fed into a carding or an air-laying system. Table 1 List of references for exposure and hazards associated with sheep's wool and synthetic fiber insulation. Subject Fiber References Exposure during manufacture Synthetic Marchant et al., 2002, Simonato et al. 1987, Enterline et al., 1983, Smith et al. 2001, Quinn et al. 2001, Dodgson et al., 1987, Esmen et al. 1978, Corn et al. 1976, Petersen and Sabroe 1991 Wool Simpson et al., 1999; Haghi, 2001; Love et al., 1988, Tonin et al. 1995, Brenton and Hallos, 1998 Exposure during installation Synthetic Marchant et al., 2002, Jacob et al. 1992, Breum et al., 2003, Petersen and Sabroe 1991 Wool – Exposure during use Synthetic – Wool – Exposure during retrofit/removal Synthetic Marchant et al., 2002 Wool – Hazards during manufacture Synthetic Marchant et al., 2002, Garry Burdett and Delphine Bard 2006, Health and Safety Executive, 2004; Baan and Grosse, 2004, Wong et al. 1991, Marsh et al., 1990; Enterline et al., 1987, Boffetta et al., 1997, Marsh et al., 2001c, Enterline et al., 1983, McDonald et al., 1990; Steenland and Stayner, 1997; Shannon et al., 1987; Chiazze et al., 1993; Marsh et al., 1996; Kjærheim et al., 2002; Sali et al., 1999; Marsh et al., 2001b; Boffetta et al., 1999; Consonni et al., 1998; Youk et al., 2001; Stone et al., 2004; Marsh et al., 2001c; Stone et al., 2001; Lipworth et al., 2009; Guber et al., 2006; Wilson et al., 1999; Sigsgaard et al., 1992; Spendlove and Fannin, 1983 Wool Burdett and Bard, 2006; Health and Safety Executive, 2004; Mastrangelo et al., 2002; Donaldson et al., 1990; Love et al., 1988; Love et al., 1991; Brown and Donaldson, 1996, Zuskin et al. 1976, Sigsgaard et al., 1992; Ozesmi et al., 1987, Zuskin et al. 1995b, Brown and Donaldson, 1991; Zuskin et al., 1993, Brown et al. 1993, Zuskin et al. 1995a, Astrakianakis et al., 2007 Hazards during installation Synthetic Marchant et al., 2002; Albin et al., 1998; Clausen et al., 1993; Baan and Grosse, 2004 Wool – Hazards during use Synthetic Burdett and Bard, 2006; Baan and Grosse, 2004; Nykter, 2006 Wool Burdett and Bard, 2006 Hazards during retrofit/removal Synthetic Marchant et al., 2002; Baan and Grosse, 2004 Wool – 404 E. Mansour et al. / Environment International 73 (2014) 402–412 In a carding system, wool passes through a number of wired rollers that essentially ‘comb’ the fibers into a single alignment. The advantage of this is higher thermal conductivity values for the final product. The resulting web of aligned fibers are then cro ss laid into several layers that overlap (Müssig, 2010), where the degree of overlap depend s on the desired density of the product (Müssig and Graupner, 2010). An air-laying system on the ot her hand is a much simpler process where the fibers are blown out onto a perforated belt forming a web of a certain density and somewhat randomly aligned fibers. This system increases production s peed and can result in a thicker final product compared to carding (Müssig, 2010). Following web formation by carding or air-laying, the web is conveyed through an oven t o allow the binder to m elt and thus provide structural integrity t o the product. During this process, the thickness is determined by setting a compression belt over the passing web to the desired measurement. Finally, the web is cut to the required length and width, and is r eady for packaging and distribution p rior to installation. 1.2.3. Installation overview Generally speaking, manufacturers of natural insulation produc ts only advise the use of dust masks during installation in dusty areas (Black Mountain Insulation, 2012; Thermafleece, 2013). Installing rolls or slabs of wool insulation is fairly straightforward and similar to tradi- tional insulation products. In some cases, netting may be used to make sure the insulation does not fall or slump off when applied in vertical wall cavities or on the underside of sloping roofs. 2. Potential health risks Manufacturing non-woven insulation material from synthetic fibers has attracted the attention of institutions and researchers regarding the health effects on production operatives, installers, and end users (see Sections 1.1 and 2.3). To date, the natural insulation industry has a small amount of available published information. In order to fully appreciate the possible health risks re sulting from the manufacture of shee p's wool insulation and due to the lack of industry-speci fic information, potential problems associated with general organic dust are firstly discussed. 2.1. Organic dust 2.1.1. Exposure to organic dust Organic dust refers to dust of plant or animal origin (Rylander, 1985), which includes dusts resulting from natural fibers and other industrial dusts such as cereal flour dust. Numerous studies address the identification of industrial natural fiber and organic dust particles, but the resul ts and definitions of the categories reported are slightly variable. One important aspect to investigate is particle size distribution and the associated effects of how long they are retained for in the body; it is logical to conclude that fibers and large particles do not reach sen- sitive tissue in the lungs and cause damage (Brenton and Hallos, 1998). Since fibers in general vary in shape and size, their settling speed and aerodynamic behavior vary accordin gly. Therefore, the aerodynamic diameter is used to categorize their atmospheric movement. For a parti- cle having a ratio of length to diameter (termed aspect ratio) ranging from 10 to 15, the aerodynamic diameter (D ae )isdefined as a function of particle diameter and density (Eq. (1)) D ae ¼ 1:6 Â D Â ffiffiffiffiffiffiffiffiffi ρ  ρ o q ð1Þ 0 5 10 15 20 25 30 35 40 Exposure to syntheƟc fibre/dust Exposure to wool fibre/dust Hazards related to syntheƟc fibre/dust Hazards related to wool fibre/dust During retrofit/removal During use During installaƟon During manufacture Fig. 2. Number of studies aimed at exposure and hazards resulting from processing synthetic and wool fiber. Fig. 3. Longitudinal section of a wool fiber showing the structural relationship between cortical and cuticular parts (Feughelman, 1997). 405E. Mansour et al. / Environment International 73 (2014) 402–412 Where D is par ticle diameter, ρ is par ticle density, and ρ o is the density of water (Breum et al., 2003). This equation can be adjusted for different aspect ratios (Gonda and Abd El Khalik, 1985). Particles having an aerodynamic diameter larg er than 30 μmare unlikely to enter the nasal passageway. The Chemical Substances Threshold Limit Values (TLV) Committee of the American Conference of Governmental Industrial Hygienists have defined the different frac- tions (American Co nference of Governmental Industrial Hygienists, 1992). Since the density of individual dust particles, and not dust in bulk, is not easy to determine, optical diameters are normally used for comparative purposes. Owen et al investigated the optical diameter of a wide range of dusts found indoors, and spec ified a rough range of optical diameters for each category (Owen et al., 1992). Table 2 summa- rizes their quote of the American Conference of Governmental Industrial Hygienist's standards on aerodynamic diameters for general particles found indoors. On the other hand, WHO sets the fiber optical diameter criterion to 3 μm or less for mineral fibers, at which it is classed as respirable. Organic fibers have a low density (about 2.7 fold lower than mineral wool), so its aerodynamic diameter is decreased. This corresponds to higher inhalability, mea ning that particles with optical diameters less than 5 μm are considered respirable (Breum et al ., 2003). Europe's equivalent for TLVs is Occupational Exposure Limits (OELs), and is used by the Health and Safety Executive (HSE). The HSE uses the definitions of BS EN 481:1993 (British Standards Institution, 1993)to determine the different fractions of dust, which are the same as TLV's; however, no diameters are assigned to any fractions as it depends on environmental factors such as air speed and breathing rate (Health and Safety Executive, 2000). According to the Control of Substances Hazardous to Health Regula- tions 2002, dust falls under the definition of a substance hazardous to health if inhalable and present at 10 mg/m 3 for 8 hours, or if respirable and present at 4 mg/m 3 for 8 hours. For wool, there exists only a long term exposure limit of 10 mg/m 3 (Health and Safety Executive, 2011). The Trades Union Congress proposes to significantly lower the limits to 2.5 mg/m 3 for all inhalable dust and to 1 mg/m 3 for respirable dust, based on an exposure of 8 hours per day (TUC, 2011). Even though, in general, it is more difficult to remove particles found in the alveolus region of the lungs as opposed to the other regions, there are several factors affecting the retention of different fractions. The actual dose received differs with the nature of the fibe r, deposition site, and other factors which deter mines its solubility (Owen et al., 1992). No studies of clearance or retention of different categories of wool dust particles have been found. 2.1.2. Hazards associated with organic dust In contrast to some studies aimed at crystalline inorganic fibers (see Section 2.3.2), no risk of mesothelioma, lung cancer, lung fibrosis or asthma has been associated with organic fibers. The reason for this logically relates to their non-durability. However, they increase the risk of obstructive lung disease and bronchitis. Studies show changes in the flow rate at low lung volumes, which is further aggravated by smoking (Zuskin et al., 1993). Overall, the possible risks are referred to as “non-specific” lung disease, which are the same effects caused by air pollution, smoking, and coalmining dust. The most contributing factor to these risks exclude particle shape, and have been identified to be: chemical additives; endotoxins present on the particle surface; microor- ganisms such as spores of fungi, actinomycetes and bacteria (Fishwick et al., 2001; Järvholm, 2000). Endotoxins refer to the outermost struc- tures anchored on a lipid membrane of gram negative bacteria, and these structures cause pathological responses (Raetz, 1990). At a manufacturing scale, dust explosions are reported to be a risk resulting from organic dust: textile, grain, paper, wood, sugar, etc. An explosion may start fro m a simple hea t source that causes a minor flame, which in turn causes a small dust cloud to form. Because the dust now occupies a large volume of air, the abundant oxygen catalyses a rapid combustion in the form of an explosion. Worst still, the pressure that results from this primary explosion agitates the dust that have previously settled on surfaces; this creates almost instantly another dust cloud that further fuels the explosion, resulting in a “domino effect”. The overall result is a large scale explosion fuelled by large amounts of dust, and is referred to a s a secondary explosion (Cashdollar and Hertzberg, 1987). Bearing in mind that the fuel is a solid and not a gas, dust particle size plays the most important role in the start and progression of an õexplosion. The National Fire Protection Association sets the maximum diameter defining a dust particle to be at 420 μm in the context of explo- sion risks. Other factors also determine the likelihood and severity of an explosion: the material's shape, porosity and moisture content, as well as surrounding f actors such as volume of the room, turbul ence a nd ignition temperature. Test methodologies exist for (1) the minimum explosible dust concentration, (2) minimum energy/temperature required to ignite a dust cloud, and (3) maximum explosion pressure, in addition to other values (Amyotte and Eckhoff, 2010). As explosions may occur with relatively large organic dust particles and due to the severity of the risk, industry has to meet high standards when processing organic material. Precautionary measures are a must, and are achieved by reducing concentr ation and removing ignition sources (The European Parliament and the Council of the Eu ropea n Union, 2000). European standards ( British Standards Institution, 2005, 2007, 2009, 2011, 2012) and those set by the National Fire Protection Association in the US (NFPA, 2013a, 2013b, 2014) are established to help industries in this area. 2.2. Wool fiber and dust It is known from the textile industry that processing natural fibers has some associated health risks. Processing cotton for example is linked to byssinosis (Burdett and Bard, 2006; Cinkotai and Whitaker, 1978; Lane et al., 2004; Simpson et al., 1998). Byssinosis is categorized exclusively as an occupational disease, and is characterized by de- creased ventilatory capacity, chest tightness, breathlessness, and other lung-related symptoms (Bouhuys et al., 1960; Valić and Žuškin, 1974). Processing sheep's wool has received some criticism from a health and safety point of view over the years, and these issues are explored in more detail below. 2.2.1. Exposure Atmospheric wool dust is generated throughout the manufacturing process of sheep's wool insulation products. Firstly, at the wool loading area where it is mixed with the binder, dust generation largely depends on how much dust the scoured wool contains in the first place. The carding and opening stages generate additional dust, as in this process fibers are subjected to heavy mechanical stress. On the other hand, an air-laying system causes small fibers and dust particles to become airborne. Cutting to the desired dimensions also contributes, although to a lesser extent. Some industries reincorporate cut-offs a nd non- conforming products thro ugh shredding and reusing the fibers with the raw materials, which again adds to the resulting atmospheric dust. Scouring companies us ually place suitable extractors and ventilation systems at key points of the process to ensure minimal atmospheric dust levels. Table 2 Owen et al.'s observations of different dust fractions and the aerodynamic diameter of most organic dusts for each category. Fraction Aerodynamic diameter (μm) Hazardous when present Inhalable fraction 5–10 at any part of the respiratory tract Thoracic fraction 1–5 within the lungs, airways and bronchiolar and alveolar tracts; i.e. beyond the larynx Respirable fraction b 1 within the alveolus; i.e. unciliated airways 406 E. Mansour et al. / Environment International 73 (2014) 402–412 There are no studies, found so far by the authors, on the exposure values related to dust resulting from processing wool for insulation ; all studies to date report on findings from the textile industry instead. Haghi categorizes the dust from a wool textile mill into four categories as stated in Table 3, but does not discuss their quantification according to these categories ( Haghi, 2001). Other studies associate cleaning and sweeping activities with the highest risk of exposure. Exposure to endoto xins at wool mills was found to be more than 4 fold less than cotton mills, and contamination decreased the more the fibers are processed (Simpson et al., 1999). 2.2.2. Hazards Most references classify wool dust as a potential allergen due to the presence of microscopic growths on its surface (Haghi, 2001; Owen et al., 1992). The dust is associated with irritation to the airways and eyes, but the e xact cause of this effect is not f ully understood (Järvholm, 2000). The HSE states that some organic dusts (see Section 2.1.1 for definition) can induce both acute and chronic inflammation in the alveolar airways and reports that exposure to wool dust induces a chronic cough and phlegm, associated with chronic bronchitis and accelerated decline in forced expiratory volume in 1 second (FEV 1 ). However, there is no explanation of the toxicological mechanisms behind these observa- tions other t han it being described as b eing complex (Health a nd Safety Executive, 2004). Other symptoms include nasal catarrh and sinusitis (Zuskin et al., 1 995), breathlessness, wheeze, an d persistent rhinitis and conjunctivitis. The latter two were noted to become less prevalent further down the textile manufacturing process (Love et al., 1988 ), showing the same trend that endotoxin co ntamination exhibits. Population studies examined the lung function of wool textile mill workers: where the inhalable fraction was greater than the recom- mended limit (10 mg/m 3 ), functional impairment was noticed in some workers, although the pattern was not consistent with current dust exposures. There is also no ev idence of the dust causing severe functional deficits or extrinsic allergic alveolitis. The conclusion of these studies is that wool dust rarely, if at all, adversely affects lung function, which is not the case of dyers and scourers using synthetic chemicals and additives (Love et al., 1991; Zuskin et al., 1997). A cross sectional survey shows that the wool industry, including scouring and combing processes, have the lowest prevalen ce of both lower and upper respiratory tract symptoms when compared to other industries such as cotton spinning and weaving, mushroom cultivation, swine confinemen t, grain handling, animal feed processing, saw mills, and poultry catching (Simpson et al., 1998). Some cases indicate that the biological and inorganic contaminants are the main cause of health risks that are comparable to cotton's (Sigsgaard et al., 1992). This is supported by reports finding unscoured wool and goat hair to greatly increase health risks. Anthrax spores are in- cluded in such reports (Watson and Keir, 1994), with occurrences in the 1960s onward, the latest being in 2011 (Kissling et al., 2012). Prolonged exposure was associated with the presence of antibodies in blood sam- ples in a dose-dependent manner, but it was not predictive. The risk of anthrax by inhalation can be considerably decreased by taking simple precautions including: use of air extractors, using personal protective equipment, and modest enhancements to ventilate the rooms. Organo- chlorine pe sticide residues in blood samples were associated with rur al sheep's wool workers in Bangalore (Dhananjayan et al., 2012). Arsenic accumulation was found in the wool of North Ronaldsay sheep, which feed off arsenic-rich seaweed; however, the arsenic and related metabo- lites were found to be easily leachable in water (Raab et al., 2002). Cases of Q fev er were also reported in the 1940s (Sigel et al., 1950). Such reports do not seem to be very common, and are not reported for scoured wool. Explosion studies are reported for three textile cases, one of them for being a sever incident at an Italian wool factory in 2001. The extent of the incident, which was heavily investigated, was attributed to second- ary explosion fuelled by about 500 kg of settled dust (Piccinini, 2008). Data on the explosivity of or ganic or inorganic fibers in general are limited, although wool's ignition temperature is known to be around 570 °C. At 250 °C, wool chars, create a non-flammable layer before it reaches its ignition temperature (Horrocks, 1996). However, laboratory tests indicate that “separation and segregation of powdered materials may change inherently non-flammable materials into flammable ones”, and that airflow dramatically increases the likelihood of an accident (Salatino et al., 2012 ). Again, depending on the nature and levels of additives in the atmo- sphere and on particles' surface, explosion risks are expected to apprecia- bly increase or decrease. Therefore, it seems prudent that manufacturing facilities undertake relevant accredited tests. 2.2.3. Toxicology A detailed study performed on the lungs of rats (Donaldson et al., 1990) shows that all dust collected from wool mills—no matter when or which part of the process it was collected from—induced inflamma- tion that peaked on the first day and did not persist beyond one week. 10 to 21% by weight of the total dust was respirable. Similar to other or- ganic dusts and studies mentioned previously, this cellular reaction was shown to be, in most cases, unrela ted to an inherent property of the dust itself; it is rather attri buted to endotoxins and other leachates present on the particles ' surface. An interesting observation was the aggregation/clumping of bronchoalveolar cells when exposed to wool dust, which tends to disappear after two weeks followin g expo sure. This phenomenon is the only one directly attributed to the particles themselves, and is no t observed with inorganic dus ts. Further investigation confirms that the observations do not result from toxicity to in vitro pulmonary epithelial cells, which are present in the airspaces of the lung (Brown and Donaldson, 1991). No apprecia- ble injury was observed by dust, their extracts, or pure endotoxins. The other possible explanation is that the dust stimulates cells to release pro-inflammatory cytokines, and this was proved to cause the inflam- mation in the short term and fibrosis of airway walls in the long term (Brown and Donaldson, 1994); although, no cases of fibroses resulting from exposure to wool dust are reported. A further study showed that both the leachates and wool particles themselves, in vivo, cause the release of tumor necrosis factor (TNF), a pleiotropic cell regulatory protein (Balkwill, 1989), in a dose dependant manner. TNF synthesis in macrophages is normally observed when endotoxins are introduced (Raetz 1990). In turn, released TNF causes cell death and e nhances inflammatory responses. Little to no difference was observed between dust collected from different sections of the manufacturing process, and le achates from wool dust resulted in a lesser effect compared to grain dust (Brown and Donaldson, 1996). There is a concern whether this further leads on to a possible carcino- genic effect (The International Agency for Research on Cancer, 1990), but recent observations indicate that exposure to endotoxins may reduce lung cancer risk by up to 40% (Astrakianakis et al., 2007) and the risk of breast cancer (Ray et al., 2007). The only available explana- tion is to attribute to endotox ins the ability to act as an agent that keeps a constant slight immune response. One study finds that a ”decrease of upper respiratory tract cancer cases and c orresponding i ncrease of lung cancer cases could be due to a lowering of dust concentration in the workplaces. So, preventive measures have paradoxically increased the lung cancer burden to the textile workers” (Mastrangelo et al., 2002). Table 3 Wool dust categorized by size as described by Haghi (Haghi, 2001). Wool dust categories Characteristics Fibers Length N 500 μm Fiber fragments Length/Width ratios b 10/1 Inorganic dust particles, silica and various silicates (i.e. soil) Diameter ≈ 20 μm Cortical cells Length ≈ 50 to 100 μm, and widths b 5 μm 407E. Mansour et al. / Environment International 73 (2014) 402–412 This effect, which may be seen as an advantage, is never observed with synthetic fibers or when dyeing is involved. Chemicals were shown to cause sinonasal cancer, as bleachers and fiber preparers exhibit an in- creased risk. When compared to dyers' trend of increasing risk of bladder cancer with increasing years of exposure, there is no evidence of a similar relationship for wool dust exposure; the risk either decreases or shows a random relationship with increasing exposure t im e. Examining the dye- ing process separately, where no textile dust is present, exposed workers exhibited chronic symptoms that include: breathlessness, chronic cough, chronic phlegm, rhinitis, sinusitis, and hoarseness, in addition to upper respiratory irritation during the work shift such as dry throat and eye irritation (Zuskin et al., 1997). This raises the question whether some studies seem to overgeneralize the problems observed in one industry, extending them to other s imilar industries that perhaps utilize different or no additives. Some studies indicate that byssinosis is related to processing wool, linking it to en dotoxin levels, but they do not m ake the distinction that wool cleaning and u se of dye s are included in t he process ( Ozesmi et al., 1987); other studies have n ot reported s uch symptoms (Spendlove and Fannin, 1983). 2.3. Comparison with synthetic fiber 2.3.1. Exposure The Occupational Safety and Health Administration (OSHA) catego- rizes dust from glass wool and mineral wool as nuisance dust, with a permissible exposure limit of 15 mg/m 3 of total dust and 5 mg/m 3 of respirable dust. There was a proposal in 1989 to set the limit at 1 fi ber/cm 3 , but it was never adopted. In 1999, the Health and Safety Partnership Program (HSPP) was established following negotiations between OSHA and trade associations. HSPP sets a voluntary permissi- ble exposure limit of 1 fiber/cm 3 for respirable synthetic vitreous fibers, based on an 8 hour exposure period (Marchant et al., 2002). On average, exposure levels to dust resulting from man-made vitre- ous fibers are less than 500,000 respirable fibers/m 3 . This was previously thought to be higher (Dodgson et al., 1987) since non-respirable frac- tions were taken into account (Baan and Grosse, 2004). Of course, some recorded exposure samples were above the 1 fiber/cm 3 recom- mendation, and that was observed in ever y industry sector with the exception of mineral wool retrofit/removal. Some applications such as separator and filtration media manufacturing and glass blowing wool without binder have much higher exposure rates compared to others, and recommendations to use respiratory protection are almost always advised (Marchant et al., 2002). 2.3.2. Hazards Almost all studies for manufacturing workers, some performed at specific groups, agree that no significant health risks arise from exposure to synthetic vitreous fibers (Shannon et a l., 1987; Enterline et al., 1987; Marsh et al., 2001b, 2001c; McDonald et al., 1990; Marsh et al., 1990, 1996; Chiazze et al., 1993; Steenland and Stayner, 1997; Consonni et al., 1998,1998;Boffetta et al., 1999; Wilson et al., 1999; Stone et al., 2001, 2004; Youk et al., 2001; Kjærheim et al., 2002; Guber et al., 2006; Lipworth et al., 2009), and attribute any observed lung cancer or mesothelioma with smoking habits (Berrigan, 2002; Buchanich et al., 2001; Marsh et al., 2001a). According to the International Agency for Research on Cancer and WHO, the evidence for carcinogenicity has been re-classed as ‘inadequate’ since 2001 after being classed as ‘limited’ in 1987 (Baan and Grosse, 2004). For non-cancer related diseases, no substantial links were observed except for exposure duration and in- creased risks of ischemic heart disease and non-malignant renal diseases (Sali et al., 1999). Other studies seem to show confl icting results and conclusions, especially when they examine installer s rather than manufacturing workers. Such studies are important since exposure to users is higher than that for producers (Marchant et al., 2002). A Danish investigation relates an increased risk of developing obstructive lung disease. The authors note a mean 26% decrease in FEV 1 that increased over five folds annually compared to a control group; the annual decline in forced vital capacity (FVC, the volume of air that can forcibly be blown out after full inspiration) was found to be over twice as fast as the control's (Fig. 4 )(Clausen et al., 1993). On the other hand, a Swedish investiga- tion indicates no effect on FVC or FEV 1 and diminish the observed effects to an increase in persistent coughing (Albin et al., 1998). Both investiga- tions studied construction workers who had similar exposure condi- tions to airborne insulation dust, and external factors such as smoking were taken into account. However, a differen ce in the mean age is noted between the two; the Swedish study purposefully looked at a younger population to exclude residual effects of asbestos exposure. However, the Danish investigation reports that “self assessed former exposure to asbestos was not associated with lung function in insulation workers”. Man-made fiber based insulation products have less microbial emis- sions than their natural-based counterparts, reaching a maximum of about 10 2 cfu/m 3 (Nykter, 2006). Therefore, health problems caused by microbial contamination are not observed at the manufacturing stage. However, it is reported that synthetic fiber insulation can support mould growth while in use ( Nielsen and Thrane, 2002), attributing to the “sick building syndrome” (Ahearn et al., 1997). 3. Conclusion Studies concerning the exposure and associated health effects related to synthetic fiber are extensively reported in the literature. These only emerged after symptoms such as breathing difficulties, which may or may not be related to synthetic dust particles, were observed post pro- duction and installation. It should be noted that installers are exposed to higher concentrations of synthetic dust particles when compared to production operatives. Although there is a push from non-legislative bodies to significantly reduce threshold limits for dust resulting from synthetic insulation, there are no established cancer risks, microbial con- tamination, or consistent lung function impairment resulting from their production or installation. Synthetic fibers may therefore appear to be relatively safe in terms of manufacturing when considering the inhala- tion and explosion risks. However anecdotal evidence suggests that health hazards are an issue during installation and long term exposure. Further studies are needed t o determine this impact. This review highlights certain limitations regarding exposure stud- ies for sheep's wool-processing ind ustries. Although an appreciable amount of research has been conducted on wool textile mills, using these exposure studies inevitably leads to an over-generalization. This is due to variability between indus trial processes between the two sectors. A key difference between manufacturers of different products is the type of wool they use: it can differ in density, structure, frag- mentation patterns, and other physical properties. Textile industries normally process soft wools, such as Merino types, whereas a wool insu- lation manufacturer would use cheaper, coarser wool regardless of color and other properties. The dust generated by different wool types will vary depending on the wool quality. Differences such as the aerody- namic diameter, distribution behavior, and settling speed will all influ- ence dust behavior. Hence, accounting for these differences is vital to ensure an undistorted understanding of exposure. This perhaps may be accomplished by categorizing different wo ol types by ran ges of densities, or by cataloguing threshold limits based on aerodynamic diameters. Other prominent differences are the processes and treatments used to manufacture a textile or an insulation product. Scouring, dyeing, and weaving, along with their associated additives differ significantly and/or are non-existent at insulation manufacturers. The literature is in agree- ment that chemicals and microorganisms play a key role in the varying health effects observed. Practical solutions might include that ensuring 408 E. Mansour et al. / Environment International 73 (2014) 402–412 the wool is scoured well, and applying treatments in a control led manner, preferably post processing. When it comes to explosion risks, although there is only one reported occurrence in the literature concerning wool, the severity is expected to be high due to secondary explosions fuelled by relatively large fibers and dust fractions. Industry does well to undertake relevant accredited tests and every precautionary measure possible, specifically in the areas of at- mospheric concentrations, additives used, and ignition sources. Needless to say, proper housekeeping is high on the list, and efforts should be made to minimize the amount of settled dust; this reduces the chances of small combustions to begin with and limits the potential propagation of secondary explosions. As far as available data go, all the above men- tioned hazards for natural fibers are factory based and can be managed to an economically viable minimum, whereas the issues with glass wool insulation are wider and still prevalent when the insulation is in the public d omain. Table 4 Summary of exposure limit values, health and explosive hazards of different dust types. General organic dust Natural wool dust Fiberglass, rock or slag wool dust Exposure limit values If inhalable and present at 10 mg/m 3 for 8 hours, or if respirable and present at 4 mg/m 3 for 8 hours (Health and Safety Executive, 2011) Long term exposure limit of 10 mg/m 3 (Health and Safety Executive, 2011) 15 mg/m 3 of total dust and 5 mg/m 3 of respirable dust (OSHA) Health hazards • Increased risk of obstructive lung disease and bronchitis and “non-specific” lung disease (Zuskin et al., 1993) • Irritation to the airways and eyes (Järvholm, 2000) • Chronic cough, phlegm, bronchitis and accelerated decline in FEV 1 (Health and Safety Executive, 2004) • Nasal catarrh and sinusitis, breathlessness and wheeze (Zuskin et al., 1995) • Increased risks of ischemic heart disease and non-malignant renal diseases (Sali et al., 1999) • Dispute over increased risk of developing obstructive lung disease (Albin et al., 1998; Clausen et al., 1993) Effect of subcomponents • Chemical additives, endotoxins and microorganisms: Both lower and upper respiratory tract symptoms (Simpson et al., 1998) • Chemical: -Functional lung impairment, breathlessness, chronic cough, chronic phlegm, rhinitis, sinusitis, hoarseness, and upper respiratory irritation (Love et al., 1991; Zuskin et al., 1997)-Organochlorine pesticide(Dhananjayan et al., 2012) and arsenic (Raab et al., 2002) residues in the blood-Byssinosis (Ozesmi et al., 1987) • Endotoxins: -Persistent rhinitis and conjunctivitis (Love et al., 1988) -Byssinosis (Ozesmi et al., 1987) • Microorganisms: -Potential allergen (Haghi, 2001; Owen et al., 1992)-Anthrax infection (Kissling et al., 2012) and Q fever (Sigel et al., 1950) if wool is unscoured • Microbial: -Exposure is less than organic dusts (Nykter, 2006), but reported to be able to support fungal growth in use(Nielsen and Thrane, 2002) which potentially causes “sick building syndrome” (Ahearn et al., 1997) Explosive hazards Present for particles having a diameter equal or less than 420 μm Risk depends on airflows, amount of dust present, segregation of dust particles, and presence of additives None reported n=340 n=114 n=114 n=166 n=59 n=59 0% 100% 200% 300% 400% 500% 600% Mean FEV1 Yearly decline in FEV1 Yearly decline in FVC Percentage relative to control group InsulaƟon workers Control group Fig. 4. Danish study (Clausen et al., 1993) findings: mean FEV 1 and yearly decline in FEV 1 and FVC (over a 6 year period) of insulation workers and a control group (bus drivers). 409E. Mansour et al. / Environment International 73 (2014) 402–412 Table 4 summarizes the exposure limit values, health hazards in- cluding the effect of subcomponents and explosive hazards of different dust types. Also, more research is needed to confirmwhetherexposuretoendo- toxins, including those found on wool dust, reduces lung and other cancer risk, and if so to investigate the underlying mechanisms. Lastly, it is worth evaluating and weighing the potential benefits versus risks that sheep's wool insulation has on human health. That would provide added comfort and value for end users, while allowing sufficient understanding to be able to adequately protect production operatives and installers. 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Precautionary measures are a must, and are achieved by reducing concentr ation and removing ignition sources