Results

PARTICIPANTS

The characteristics of the populations under study and the response rates are shown in table 1. The overall response rate was 80%. The groups were not well matched for several of the variables. The tappers were older than the glove manufacturers who were older than the college students. There were more men than women among tappers than the other groups. There were few smokers in any group; college students were less atopic than the other groups, therefore at least smoking was well matched. Table 2 shows characteristics of the participating glove factory workers by exposure group. Most workers were in the high exposure group (64.6%). The mean age was similar in the three exposure groups. There were more women in the high exposure group and more smokers and atopic people in the low exposure group. Workers in all exposure groups had similar duration of exposure.

MEASUREMENTS OF LATEX IN AIR

The air measurements were not normally distributed and geometric mean values by exposure group are shown in table 3. The result confirmed the correct categorisation of workers into the three different exposure groups. The concentrations were all high compared with areas of high powdered glove use in the United Kingdom hospital. The tappers had similar concentrations of exposure in air to the moderate group of glove manufacturers.

PREVALENCE OF POSITIVE SKIN PRICK TEST TO NRL

Among those surveyed 96% of glove factory workers and all tappers and unexposed college students underwent skin prick testing. Among 795 skin prick tested workers, 12 (1.5%, 95% CI 0.67 to 2.35) workers had a positive reaction to the latex reagent. These comprised eight glove factory workers (1.7%, 95% CI 0.5 to 2.8) and four tappers (1.3%, 95% CI 0.5 to 2.0). None of the college students showed a positive reaction. The percentages with wheals of more than 2 mm and 3 mm are shown in table 4. There were no significant relations between wheal diameter and any of the exposure groups, smoking, age, sex, or family history of allergy. There were significantly more 2 mm reactors who were atopic (10.6% v 6.3%, χ² ₁=4.46, p=0.04), however, most workers sensitised to latex had a negative reaction to grass pollen. Although not significant, the prevalence of a >2 mm wheal to latex increased with duration of employment in the glove manufacturers from 6.8% for those employed for less than 1 year, to 9.5% for those employed for 1–5 years and 11.3% for those employed for more than 5 years. No relation between duration of employment and latex wheal positivity was found in the plantation workers.

PREVALENCE OF WORK RELATED SYMPTOMS

Work related symptoms were common in all groups, and particularly common in the college students (table 5). There were no significant differences between the exposure groups for any symptom. The low exposure glove manufacturers were the least symptomatic for breathlessness, cough, eye irritation, runny nose, and urticaria.

Discussion

This is the first study of a representative sample of latex glove factory and plantation workers in which aeroallergen measurements and a standardised natural rubber latex allergen extract were used. The response rate was high. The latex allergen concentrations in air were high,25 but despite this the rate of sensitisation, as assessed by skin prick testing, was very low.

Stallergènes latex skin prick test reagents were used in this study. This reagent has been standardised and the concentration of 1:200 w/v, showed a 93% sensitivity and 100% specificity, 100% negative predictive value (NPV), and 96% positive predictive value (PPV).26 Turjanmaa et al 26 stated that it contains most of the important allergens including proteins of about 14, 20, 27, 30, and 45 kDa. It should therefore be able to detect latex sensitisation if present. The ability to produce a wheal response in the Thai population was shown by the positive histamine wheals and their response to common environmental allergens. The largest wheal of latex reaction was 4.2 mm. The same bottle of latex skin prick tests produced wheals up to 8 mm in United Kingdom hospital workers, where 14.6% of those tested had at least a 3 mm wheal.27 If the criterion for a positive test was relaxed to greater than 2 mm, a positive test occurred in 8.1% of glove factory workers, 6.7% of tappers, and 1.4% of college students. These figures are still less than the 11% found in the Canadian glove factory reported by Tarlo et al 22 who used an unstandardised allergen and reported wheals greater than 2 mm as positive. Even lower results have been reported from Malaysia, where the prevalence of a 2 mm or greater wheal among glove factory workers was 2%23 and 4.7% among tappers (Shahnaz M, personal communication). The latex skin test reagent used in the Malaysian study was also unstandardised.

The Thai workers studied were exposed to high concentrations of latex in air with median concentrations 30 times higher than in the United Kingdom hospital studied,25 and yet had a low prevalence of strongly positive skin prick tests to latex. This difference highlights the need to consider other factors which may be different between the Thai workers and the United Kingdom hospital staff. Most sensitised healthcare workers have had extensive skin exposure to latex, which is likely to be the predominant route of sensitisation in them. Occlusive skin exposure did not occur in the Thai workers studied. Once sensitised healthcare workers often react to airborne latex resulting in rhinitis and asthma. This may include the nature of the allergens in Thailand and the ability of the Thai workers to form IgE antibodies to latex allergens. It is possible that very high concentrations of exposure lead to a degree of tolerance and that lower concentrations might paradoxically be associated with greater levels of sensitisation. Factors which may reduce atopy are infection, poor hygiene, and large family size, and a factor which might block the IgE response is high concentrations of IgE in response to parasitic infection.

The low level of sensitisation in the current study may be related to the short period of exposure of many of the workers. Most glove factory workers had been exposed to latex for less than 1 year. The prevalence of latex sensitisation (wheal >2 mm diameter) increased with increasing duration of exposure in the glove manufacturers. The highest prevalence of latex sensitisation was found among those exposed between 5 and 10 years (11.3%). As about 50% of workers had been exposed to latex for 1 year or less, the present study is likely to have underestimated potential sensitisation. By contrast, Azizahet al 23 found no relation between the duration of time the workers spent in latex glove factories and the presence of sensitisation to latex defined by a positive skin prick test to latex. However, the workers in that study were not randomly or systematically selected and as such, the results reported might not be representative. Sensitisation may develop soon after first exposure; for example workers in the sanding section of a doll factory all developed latex allergic symptoms with the positive skin prick tests confirmation within 3 years.28 Although the exposure times were much higher among the rubber tappers than among the glove factory workers in the present study, the prevalence of latex sensitisation was also small and even smaller than among the glove factory workers. This implies that the duration of exposure alone may not explain the low sensitivity to latex among the Thai workers exposed to it.

The relation between the prevalence of sensitisation and exposure to latex allergens can be modified by host factors. Host factors related to atopy are associated with latex sensitisation.16 17 29-32 In the present study, the prevalence of atopy defined by skin prick tests positive to one or more common inhalable allergens was 29.7% in the Thai workers, which was significantly more than the Thai college students (13.9%). The prevalence of atopy seemed to increase with increasing age unlike other studies,33-35 which reported the reverse effect. The prevalence of atopy in Thai workers seems to be close to the general prevalence of atopy in the United Kingdom working population, which is 25% to 33%.36 Unfortunately, data on the prevalence of atopy among the Thai working population has not been reported; therefore comparison can only be made with other groups of the Thai population. The prevalence of atopy, defined as having a history of atopic disease and positive skin prick tests to common inhalable allergens, in the general population in north and north east Thailand was about 20%.37Among the Thai latex workers studied who had a positive skin prick test to latex (wheal >2 mm), 42% had atopy. This compares with other latex sensitisation studies which reported that 64% to 85% of the subjects sensitised to latex were atopic.

The latex particle size may be different in the Thai workers than the healthcare workers, where corn starch powder is the major carrier of latex allergens. Little is known of any variation in particle size of the corn starch between different manufacturers. The two studies which have measured particle size have produced inconsistent results; one showing that the corn starch was 1–3 μm38 and the other that particles carrying latex allergens were mainly greater than 7 μm.39 In hospital studies the latex allergen has been shown to be tightly bound to corn starch.38Replacement of talc, to which latex is less firmly bound, is one of the possible explanations for the increasing prevalence of latex allergy.40

There are no standardised respiratory or occupational questionnaires available in Thai. The questionnaire used in this study was developed in English, and produced results indicating increasing work related symptoms with increasing exposure to latex gloves in a United Kingdom hospital. It is possible that the questions were interpreted in a different manner by the Thai subjects studied. In particular the college students came from an area without rubber plantations. They were studying commerce, home economics, and textiles and were not exposed to latex products at college; despite this they had the highest level of work related symptoms. Glove manufacturers are exposed to chlorine and tappers to acetylene from their headlamps (tapping is done at night), providing alternative causes for some of their symptoms. The group with medium exposure in air had more symptoms than those with high exposure. This could be due to selective loss of symptomatic workers from the high exposure group but it is more likely to be due to the lack of specificity of the questionnaire as symptoms were usually high in the unexposed college students. A study on Malaysian workers in a latex glove factory found that none of the workers sensitised to latex had any adverse reactions on exposure to latex.23Lagier et al 5 have also reported a poor correlation between at least one self reported symptom related to latex gloves and response to skin prick tests with only a positive predictive value of only 18.6%. Therefore, the findings of the present study and other previous reports suggest that self reported symptoms may be non-specific for latex sensitisation.

Conclusion

The study shows that sensitisation to well defined latex allergen extract was infrequent among the Thai workers studied despite concentrations in air much higher than in the United Kingdom hospital. It is likely that sensitisation is often by cutaneous exposure but that once sensitisation has occurred air concentrations of latex induce asthma and rhinitis. Control of air concentrations alone, for instance by limiting powder in the hospital, is unlikely to control sensitisation in healthcare workers without also reducing skin absorption by reducing extractable antigen from latex gloves.