Occupational Asthma in Korea

Asthma is a disease presenting with reversible and repeated symptoms, such as wheezing, dyspnea, chest tightness and cough due to increased bronchial responsiveness from chronic inflammation in the respiratory tract (1). Occupational asthma (OA) is a type of asthma originating from the working environment, and is the most common occupational respiratory disease (2). OA is especially important because it can lead to respiratory disturbance and restrict economic activity by developing in a young and healthy population. More than 300 causative agents of OA have been documented (3), and the number is increasing due to the introduction of new chemicals. The detection of occupational causative agents and reduction or elimination of exposure to such agents can prevent OA.

Although estimates vary, occupation-related adult-onset asthma in a normal population was reported to be 2% in the United States (4) and 15% in Japan (5), and a recent study in 13 European countries reported an OA prevalence of 10-25% (6). A recent systematic analysis of all reports in the world indicated that 15-20% of adult-onset asthma was caused by occupational exposure (7). In Korea, new cases of OA are being reported every year since the first OA case due to inhaled polyurethane was reported in 1978, and a study at a university hospital reported 3.9% of adult-onset asthma cases were OA (8).

The aim of this study was to examine the incidence and the trend of OA in Korea for 15 yr (1992-2006) through compensation data from the Industrial Accident Compensation Insurance (IACI) Act and regional and national OA surveillance system data; and to understand the causative agents and prevalence of OA in Korea through literature and deliberations of occupational cases by the Occupational Safety and Health Research Institute (OSHRI).

Since Ramazzini, the father of Occupational Medicine, observed and reported shortness of breath and urticaria in grain carriers exposed to organic dust in 1713, more than 300 OA causative agents have been determined (3). In Korean workplaces, all of these agents are widely used. In addition, new techniques and chemicals continuously introduced to workplaces will maintain the concern for OA.

Both the IACI and the surveillance system data demonstrated very low incidence rates of OA in this study. According to IACI data, the incidence the was about 10 per million workers until the late 1990s, increased to approximately 20 in 2000s, and showed a downward trend again. This may be because exposure levels of hazardous substances have decreased recently, owing to improved working environment in spite of various kinds of hazardous substances. In addition, substitution for hazardous substances, e.g., substituting TDI, a major asthma causative agent, for less reactive methylene diphenyl diisocyanate (MDI) may be another reason for the downward trend. A review reported a clear downward trend of TDI-induced asthma worldwide (9). However, in Korea, it is hard to conclude that a decrease in cases according to IACI data reflects an actual decrease in OA, because the number of cases reported by the surveillance system during the same period was greater when compared to the IACI data, and the surveillance system data showed an upward trend.

IACI data showed low incidence of OA probably because only workers working at workplaces covered by ICAI could apply for compensation, leading to underestimation because workers not covered would be missed. However, this cannot fully explain the low incidence, considering that cases by the IACI decreased even after the expansion of its coverage in 2000. Therefore, there might be other more plausible reasons, including workers' ignorance of IACI and relevant procedures, fear of losing their job, self-employed workers, or agreements with the employer (10). In addition, with varying asthma symptom severity, mild cases were likely to go unreported.

As shown above, though IACI data are useful, they have limitations due to various socioeconomic and medical factors, and low claiming rate. Therefore, some countries operate a surveillance system, which has proved to be very effective. In Korea, regional surveillance systems for OA have started since 1999, and the national OA Surveillance System, which is based on voluntary reporting, was initiated in 1998 (10). However, a voluntary surveillance system requires understanding of OA and active participation of clinicians. Korean surveillance system data greatly differ from data of other countries, indicating the need for vitalization of the surveillance system in Korea.

This study reveals that the most important OA causative agent in Korea was isocyanates. Although with different appearances in countries, isocyanates were reported as the major OA causative agent in most countries (11-13). This is attributable to the use of isocyanates in a variety of products, (e.g., car sheets, cushions, sofas, beds, insulating material, glue, and polyurethane paint) owing to their very high reactivity, which makes them important intermediates in producing polyurethane (14). However, in Korea there may be another reason: The diagnostic criteria of occupational diseases by the IACI Act specify 'asthma by isocyanates'. In other words, under present regulations, asthma occurring during work with isocyanates may be recognized right away without epidemiological investigation, whereas asthma from hazards other than isocyanates is examined by complicated epidemiological investigation.

In Korea, reactive dye was the second most frequently reported agent, which differed from the results of other countries. This may be due to collective application for recognition of occupational diseases by some workplaces in some regions. However, reactive dye is also highly reactive, readily scattered, and frequently used in open-space worksites, and therefore may affect nearby neighborhoods. Asthma cases in workers who have worked near a reactive dye manufacturing factory were actually reported in Korea, indicating that careful exposure control is necessary (15).

While wheat flour and grain dust were major causes of OA in other countries, this was not the case with Korea. It may be because clinicians are reluctant to report upon relatively unknown hazardous factors without definite evidence, or workers may not apply for recognition of occupational diseases due to lack of information. However, recent surveillance system data report many cases of OA due to wheat flour dust and pharmaceutical dust. Therefore, it appears that understanding of asthma causative agents is gradually expanding.

In Korea, various chemicals, heavy metal dust and other dust, isocyanates and reactive dye have induced OA. At 34,797 manufacturing workplaces in Korea, 244,420 workers were exposed to dust or fumes (16). Accordingly, it has been estimated that various chemicals and heavy metal dust have induced a substantial number of OA cases. However, cases actually compensated as occupational diseases were very rare.

In prevalence studies of OA in Korea, prevalence and causative agents differed very much; for example, prevalence of TDI-induced OA varied greatly over 0.58-21.6%. This may be due to different reactivity of agents, different exposure levels and working environment for each asthma agent, as well as differing OA diagnosis criteria used.

In conclusion, more OA should be discovered through systematic and active surveillance systems, and workers exposed to actual hazards should be better protected by early detection and intervention, such as prompt avoidance of causative agents, and improvements in the working environment.