Some environmental exposures have rather consistently 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 parental smoking is very likely to be causally related to acute lower respiratory tract illnesses in infancy and to childhood asthma and wheezing. A number of studies have also shown that active smoking is associated with the onset of asthma in adolescents and adults.