Glass microfiber is one of the synthetic man-made vitreous fibers that are widely used for insulation. Because of the controversy concerning potential non-malignant health effects of glass microfibers, we studied workers in a factory producing glass microfiber sheets in Thailand. Factory workers experienced increased risk of cough, wheezing, breathlessness, nasal symptoms, skin symptoms and ever asthma compared to office workers. After adjusting for potential confounders, the risk of breathlessness and skin symptoms remained significantly increased, and the risk of both of these symptoms showed a trend suggesting exposure-response relation with increasing glass microfiber exposure level. Interestingly, these effects were detected at concentrations of respirable dust, consisting mainly of glass microfibers, that were below the threshold limit value for respirable dust of 3 mg/m3.
The respiratory and skin effects observed could be due to an irritant mechanism by glass microfibers. Lack of effect on spirometric lung function would be consistent with an irritant effect. A review by the World Health Organization (WHO) suggested that MMVF can cause transient irritation of skin, eyes and upper airways, but that there has been so far insufficient evidence of effects on lower airways . In our study, the risks of wheezing (adjusted OR 2.20) and asthma (adjusted OR 3.51) were also increased, although not statistically significantly, which raises the question whether exposure to glass microfibers could induce an irritant-type of asthma . Our study is not able to answer this question definitely, because of the small number of asthmatics in our study population. Abbate and co-workers  investigated 29 men employed in glass fiber reinforced plastic processing with a clinical check-up and bronchoalveolar lavage (BAL). Microscopic and biochemical analyses of BAL fluid suggested that oxidative stress activates an inflammatory process in the airways as a consequence of exposure to glass fibers.
Some processes of the glass microfiber production involve working with potential sensitizers, such as formaldehyde. Exposure to sensitizing chemicals was associated with significantly increased risks of cough and nasal symptoms. The risk of breathlessness was also increased, but no obvious effect on skin symptoms was detected. Exposure to sensitizing chemicals did not affect adversely spirometric lung function or the risk of asthma, but these conclusions have to be cautious because of the small number of workers with sensitizer exposure (n = 19).
The small sample size of office workers is a limitation of this study. The number of subjects that could be studied was dependent on access to workforces, a problem commonly encountered in occupational epidemiology. To counterbalance this limitation, we made efforts to get good response rates and succeeded in this well. The response rate was 100% among glass microfiber factory workers currently employed at the factory and it was also relatively good at 73% among office workers, so selection bias is not likely to explain our results.
The outcome assessment in this study was based on self-report of symptoms and doctor-diagnosed asthma and measurements of lung function. All of these investigations were carried out in the same way in factory and office workers. The same protocol for the questionnaire and spirometry was applied in all four factories, where office workers were recruited to form the control group. It is possible that our study underestimates to some extent the true effects of glass microfiber exposure because of the cross-sectional study design. It has previously been estimated that about 5% of workers involved in MMVF production leave employment because of problems of skin irritation , meaning that those staying in the industry are likely to be selected based on better health. Our finding that vital capacity was actually somewhat better in factory workers than in office workers supports the possibility of some 'healthy worker' selection taking place. A small number of asthmatics, especially current asthmatics among office workers, is a limitation of the study, as the results suggested increased risk of ever asthma in relation to glass microfiber exposure, but this did not reach statistical significance. The rather small number detected also among factory workers, although the risk of wheezing was increased, may reflect the health care system. Workers may not seek medical help for their asthma-type symptoms because of fear of losing their job.
Exposure assessment was based on job title complemented by air respirable dust measurements from 2004 and 2005. There had not been any changes in the industrial processes or environmental control measures between these years and our data collection in 2006, so the air measurements are likely to reflect well the relevant exposure situation in the factory. These methods give a rough estimate of microfiber exposure level, which enabled us to compare high and low exposure groups. The unexposed control group (i.e. office workers) was formed of those working for the companies, but not working in the production areas, packing areas, warehouse or quality control, i.e. not having any major exposures. However, the managers sometimes visited the process for short periods of time. During these visits they were required to wear protective masks. It is possible that some of them could have had small amount of exposure, which could cause some underestimation of the true effects of glass microfiber exposure.
The number of workers exposed to sensitizing chemicals was small (n = 19), but such exposure showed significant associations with cough and nasal symptoms. Sensitizer exposure was related to a somewhat different symptom pattern than glass microfiber exposure, but most of those exposed to sensitizers were also exposed to microfibers, so we were not able to disentangle their effects sharply.
We collected data on many potential confounders in our questionnaire, including personal characteristics (sex, age), genetic background (parental atopy or asthma), socio-economic status (education), lifestyle habits (smoking), and other risk factors at work (SHS exposure, stress at work) and at home (pet keeping). We addressed all of these and controlled for the factors that were important potential confounders in the multivariate regression models in order to exclude them as potential explanations for our findings.
Synthesis with previous knowledge
Only a few previous studies have been reported on non-cancer respiratory effects of glass fibers and they have provided inconsistent results. According to reviews there has been insufficient evidence to make any firm conclusions , so more studies on this topic in human subjects are needed.
In the only previous epidemiological study on respiratory effects, Moulin and co-workers  investigated 2,024 men from three plants manufacturing glass wool and two manufacturing rock wool in France. In the biggest plant studied, exposure to fibers was related to significantly increased risk of cough, phlegm, dyspnea and pharynx-larynx symptoms, ORs being 1.6–6.0. However, no effects were detected in the other four factories. Working in resin preparation was consistently related to the risk of pharynx-larynx symptoms. Thus, the findings in the largest factory are consistent with our findings.
Results of three clinical studies on non-malignant respiratory effects in workers exposed to glass fibers are partly compatible with our results. A study of workers of a filament glass fiber plant in UK identified 7 cases who had work-related asthma in serial PEF measurements, but was not able to identify the causal agent in challenge testing . An investigation of a fiberglass wool insulation production facility in USA was prompted because of high prevalence of wheezing and use of asthma medication among employees, but these problems seemed to be related to endotoxin from bacteria growing in the recirculating wash water . Kilburn and co-workers  studied clinically 284 workers from Sheet metal Workers' International Association who had at least 20 years of exposure to fiberglass. Nineteen percent of the study population reported throat irritation, 13% nose irritation and 10% chest burning. Spirometric lung function was reduced in fiberglass-exposed workers compared to the reference values. However, some of these study subjects had also had exposure to asbestos and there was no control population for comparison to assess the risks of these conditions.
Some studies have investigated mortality from non-malignant respiratory disease in workers exposed to MMVF. Chiazze and coworkers  conducted a case-control study among workers employed at the Owens-Corming Fiberglass plant in Ohio and reported a non-significantly increased adjusted OR 1.50 (95% CI 0.55–4.08) in relation to cumulative exposure to >300 fibers/ml. A longitudinal study of MMVF workers from 13 factories in seven countries followed 11,373 workers and found a slightly increased mortality from bronchitis, emphysema and asthma among glass wool workers with a SMR 1.12 (95% CI 0.82–1.49) .
Workers are also exposed to glass microfibers through direct contact with the skin. Skin symptoms, such as itching, and dermatitis are rather consistently reported in relation to occupational exposure to glass microfibers or other MMVFs in case reports, studies of individual factories, and registry-based studies [10–12, 32]. Glass fibers and sensitizers, such as phenol-formaldehyde resin, have been identified as causes for individual cases. According to the Finnish Register of Occupational Diseases, the rate of skin diseases due to MMVF was highest in insulation workers with an annual incidence of 9.1 per 100,000 employed workers . Only one previous study assessed the risk of skin diseases related to glass microfiber exposure. A Japanese cross-sectional study of 148 workers from fiberglass reinforced plastics factories  found significantly increased risk of work-related skin problems with OR ranging between 4.10 and 4.69. Thus, the Japanese study is consistent with our study that shows an OR of 3.89 for skin symptoms in factory workers compared to office workers.