Overall scale of disease including trends
Occupational asthma occurs when workplace exposures to particular substances result in a biological change in a person's airways, known as the hypersensitive state, so that subsequent exposure to the same substance triggers an asthma attack. The mechanism by which this happens varies from one substance to another. Because the range of industries which use substances with the potential to cause asthma is quite broad, and not all employees in these industries will necessarily be exposed, it is difficult to estimate with any confidence the total number of workers at risk.
Surveys of Self-reported Work-related Illness provide estimates of the prevalence of occupational illnesses – meaning, the total number of people ill at any given time. From the latest survey, it was estimated that in 2005/06 156 000 people who have ever worked had “breathing or lung problems” caused or made worse by work (95% Confidence Interval: 139 000-174 000) [see SWI Table 3. This estimate includes all categories of respiratory disease. In the corresponding 1995 survey, which collected more detailed information about diseases categories, about three-quarters of those reporting work related lower respiratory disease described symptoms consistent with asthma. However, since this data relied on self diagnosis it should be treated with caution. The responses given were obviously dependent on lay people’s perceptions of medical matters and, as such, cannot necessarily be taken directly as an indicator of the ‘true’ extent of work-related asthma. This 1995 survey also found that around 90% of people with a lower respiratory disease reported that it was caused by inhaling fumes, dusts or other harmful substances [1]. The general comments made regarding self-reported work-related disease apply to these figures.
Table IIS08 shows DWP Industrial Injuries Scheme (IIS) data on disablement benefit for occupational asthma. Benefit first became payable for this disease in 1982 in respect of a specified list of substances, initially only agents 1 to 7. This list of specific substances was extended (by the addition of agents 8 to 14) in 1986 and again (agents 15 to 24) in 1991 when the addition included an 'open category' which allows benefit to be paid for occupational asthma caused by an agent not specifically listed, provided a causal link is proven in each case. The list has remained constant for the period shown in Figure 1 below with one minor exception – the addition of latex to the list of agents in March 2005. However this has had little impact on the overall numbers, with only 5 cases caused by latex in 2005. The considerable fall in the number of cases assessed, in 1997 and 1998, may be due to changes in DWP data collection procedures, which took effect in the course of 1997 and which continues to affect levels of reporting of assessed cases. The number of cases assessed in 2005 was 230, an increase of around 20% compared to the previous two years.
A better indication of the overall incidence of occupational asthma can be obtained from the reports of new cases of the disease seen by the physicians who participate in the Surveillance of Work-related and Occupational Respiratory Disease (SWORD) and Occupational Physicians Reporting Activity (OPRA), part of The Health and Occupation Reporting network (THOR) . Numbers of cases from both these schemes are shown in Figure 1, along with the IIS data, and in Table THORR01 and THORR02, along with SWORD and OPRA data for other respiratory diseases.
Over the past three years, SWORD/OPRA has recorded nearly three times as many cases of occupational asthma in Great Britain (1714) as were assessed under the Industrial Injuries Scheme (610). There are a number of factors which may account for the differences between the IIS and SWORD/OPRA data. Since the IIS only covers asthma cases due to exposure to one of the specified agents or where individual proof of an occupational cause is determined, the scheme tends not to pick up cases arising from substances or in occupational settings where the link with asthma is less well established or well known. Furthermore, the self-employed are not covered by the IIS and level of compensation available for even those who are severely disabled may not provide sufficient incentive for all elligible individuals to apply.
Figures from SWORD and OPRA also underestimate the incidence of occupational asthma. A previous review of the data [2] suggested that SWORD may underestimate the true incidence of consultant-diagnosed asthma by at least a third. Moreover, as many cases of occupational respiratory disease will not be referred to a consultant physician, the total incidence of the disease is likely to be several times higher than the figure from SWORD. Indeed a reasonably conservative estimate would be that the total incidence of occupationally-related disease is 3 to 5 times the incidence of consultant-diagnosed asthma, as measured by SWORD, that is, around 1500-2500 annual cases. The incidence based on this narrower definition of occupational asthma is much lower than the estimated incidence of “breathing or lung problems” from the 2005/06 Self-reported Work-related Illness Survey of 23,000 cases (95% CI: 16,000-30,000) [See SWI Table 6].
As Figure 1 shows, there have been year-on-year fluctuations in the numbers of new cases reported to SWORD/OPRA. Apparent trends in the overall number of cases can be misleading because the figures are affected by changes in levels of participation in the scheme over time and other sources of variation, such as a tendency for reporters to include more cases than they should when they first start reporting and fewer cases than they should once they have been reporting for some time (so called “reporter fatigue”), and seasonal effects associated with the time of year physicians report to the scheme. However, statistical modelling by the University of Manchester to take into account these effects identified a statistically significant downward trend from 1999 and suggest that the apparent trend over this period seen in Figure 1 is in fact real.
Both SWORD and DWP Industrial Injuries scheme figures continue to implicate isocyanates as the chemical group responsible for the highest proportion of new cases of occupational asthma, as they have for some years. Both sources also continue to show flour/grain as the next most commonly cited agent. Data from SWORD suggests that approximately three quarters of such cases are attributable to flour and one quarter to grain. Table THORR07 shows a full breakdown of SWORD/OPRA cases by agent, as does Table IIS09 .
Industrial and occupational analyses of SWORD/OPRA cases can give some insight into the types of workplaces and activities that are currently causing occupational asthma in the British workforce. Table THORR05 and Table THORR06 show the average number of SWORD and OPRA cases reported per year during the period 2003-2005, by occupation and industry respectively, together with estimated rates per 100 000 workers. These latter rates are calculated by using a denominator based on the number of workers identified in the Labour Force Survey in the relevant occupational or industrial sector. Thus the denominator is representative of the whole sector whereas the number of cases reported is limited by underreporting (see above). As a consequence the rates identified should be seen as minimal estimates.
Because the coverage of British industry by occupational physicians varies by type of industry and occupation the chest physician (SWORD) data alone should be used for making comparative statements between different industries and occupations. Given that there is not thought to be a great deal of overlap in cases reported in the two schemes, data from both chest physicians (SWORD) and occupational physicians (OPRA) can be combined to give the most complete available estimate for any particular subgroup.
Adopting this approach, Table THORR05 shows that the occupational categories ‘moulders, core makers, die casters' (109 cases per 100 000 per year), ‘vehicle spray painters’ (96 cases per 100 000 per year), and ‘bakers, flour confectioners’ (78 cases per 100 000 per year) have the highest rates of occupational asthma as seen by chest physicians. One noteworthy aspect of these figures is that the incidence rate for vehicle spray painters has dropped by over 40% when the figures for 2003-2005 are compared with 2001-2003. As explained above, the rates are quoted for the purposes of comparison only and, because they exclude reports from occupational physicians, are in all cases underestimates. The best estimate of the rate for any one specific occupation, to be looked at in isolation, would include reports from both types of physician, although this will still be an underestimate. For example, our best estimate of the rate for vehicle spray painters would be 139 cases per 100 000 workers per year as opposed to the 96 quoted above.
Considering the data in Table THORR06 in the same way shows that the individual industry division with the highest rate of occupational asthma as seen by chest physicians is the 'manufacture of motor vehicles, trailers and semi-trailers' (24 cases per 100 000 workers). The next highest rates are in the 'manufacture of basic metals' (20 cases per 100 000 workers per year).
Table THORR03 and Table THORR04 show the distribution of the cases of occupational respiratory disease reported to SWORD and OPRA by age and by country, respectively. For both schemes, Table THORR03 shows that the most common age groups for new cases of occupational asthma are 35-44 and 45-54 years for males (with around one-quarter of the total each), and 25-34 and 35-44 years for females (again with around one-quarter of the total each). Table THORR04 shows that 86 per cent of reported occupational asthma cases were in England, with 9 per cent in Scotland and 5 per cent in Wales.
References
- Self-reported work-related illness in 1995, Jones et al. HSE Books (1998), ISBN 071761509X. Available from HSE books
- Self-reported work-related illness in 2004/05, Jones et al (2006)
- Meredith S and Nordman H 'Occupational asthma: measures of frequency from four countries' Thorax 1996 51 435-440.
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