Population-based studies are consistent in reporting an accelerated rate of decline in FEV, in adult asthmatic subjects relative to normal individuals (Table 8).* With the exception of work by Fletcher et al, these studies do not adjust for baseline level of lung function, nor do they consider possible interactive effects of asthma with other symptoms.
Longitudinal data from a study of miners suggest that increased airway responsiveness is associated with an accelerated decline in FEV, and a greater development of respiratory symptoms. It is unclear from existing studies how disease severity influences prognosis. Existing population data would suggest that asthma with normal lung (unction evolves to asthma with some degree of fixed air flow obstruction; however, conclusive data demonstrating this point are not available. Burrows has suggested that subjects with asthma have a better prognosis in terms of mortality and decline in FEV, than subjects with emphysema. Whether this reflects selection bias or treatment effects is unknown.
Occupational asthma is a disorder triggered by many substances. Over 200 compounds have been described as causing occupational asthma. High molecular weight compounds tend to be associated with specific IgE antibodies. Low molecular weight compounds are rarely associated with specific antibodies. The natural history of each of these clinical syndromes is incompletely described at best.

Table 8—Relationship oj Asthma to Decline tn Ftvl

Study Definition of Asthma Length of Follow-up(yr) Rate of Decline Greater Than Control (ml/yr)
Fletcher et al Doctors diagnosis of asthma 8 22
Schachter et al Have you ever had asthma? 6 18
Peat et al Doctors diagnosis and attacks of shortness of breath with wheeze 18 15
Buist and Vollmer Doctor s diagnosis of asthma 9-11 0