Some environmental exposures have rather con- sistently been shown to influence the incidence of asthma. The most consistent association is that with passive or active exposure to tobacco smoke. A thorough meta-analysis has concluded that pa- rental smoking is very likely to be causally related to acute lower respiratory tract illnesses in in- fancy and to childhood asthma and wheezing.61 A number of studies have also shown that active smoking is associated with the onset of asthma in adolescents and adults.62-64