Trends in incidence

Statistics based on reports of occupational asthma within the THOR scheme are affected by various factors including the number and type of participating specialist and occupational physicians, their reporting habits, and by seasonal effects associated with the time of year they report.  This makes assessment of trends based on total annual estimated cases problematic since these factors - as well as the true incidence - can vary over time.  However, statistical modelling by the University of Manchester showed statistically significant downward tends in the incidence of occupational asthma over the period 1999-2006 after taking account of some of these effects.  However, these trends are driven by a large reduction in incidence in 2000 followed by a period over which the incidence remained more constant. The analyses do not take account of a possible tendency for reporters to include fewer cases than they should once they have been reporting for some time (so called "reporting fatigue") since the appropriate methodology is still being developed.  If the data were affected by reporting fatigue this would tend to reduce any observed downward trends.  Annual estimated cases of occupational asthma based on the THOR scheme are shown in Figure 1 below. 

The current list of agents for which benefit is payable within the IIDB scheme has remained constant for the period shown in Figure 1 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 and less than 5 in 2006 (Table IIDB08). During the first part of the period shown in Figure 1, the annual number of assessed cases fell quite strongly – particularly during 1997 and 1998. This may be due to changes in DWP data collection procedures, which took effect in the course of 1997 and which continue to affect levels of reporting of assessed cases. After a gradual rise in annual cases since 2001, the number fell in 2006 by 30% compared to the previous year.