Bronchial response to inhaled histamine in asymptomatic young smokers

Non-specific bronchial response to inhaled histamine was measured in 21 young (21.2 +/- 1.9 years) asymptomatic smokers (2.9 +/- 2.1 pack years) and in 21 age, sex, and height matched asymptomatic controls. Following a 2 min inhalation of histamine 8 mg/ml, the smokers showed smaller reduction in FEV1 (3.4 +/- 3.3% vs 7.0 +/- 6.2%, p less than 0.05) and in MMF (7.6 +/- 15.6% vs 21.1 +/- 17.3%, p less than 0.02) than did the non-smokers. The threshold concentration of histamine was defined as that concentration where FEV1 consistently was greater than 2 SD below the mean of that obtained from 5 to 7 prehistamine spirograms. A histamine threshold between 2 and 8 mg/ml was detectable in seven smokers, and a threshold between 1 and 8 mg/ml was seen in 14 non-smokers. Histamine threshold was not measurable (i.e. greater than 8 mg/ml) in 14 smokers and seven non-smokers (p less than 0.05). Thus, these young asymptomatic smokers of brief duration failed to show increased response to histamine; in fact, they appeared to respond less to histamine than did matched controls. This may be the result of selection (who chooses to smoke) or alternatively it may be due to an acquired loss of sensitivity (development of tolerance) to inhaled irritants.