Histamine responsiveness. Mean FEV1 as the percentage of the initial value (0) after increasing doses of inhaled histamine, administered by aerosol by a hand held nebuliser, according to exposure group. Error bars have been deleted for clarity. * denotes significant difference (p < 0.05) compared with controls. At the higher doses the numbers of subjects are slightly lower than indicated in the legend because the test was interrupted when FEV1 decreased by 20% or more (i.e. detectable PD20, see table 2 for the number of subjects with a detectable PD20 in each group).