Vitamin C and asthma in children: modification of the effect by age, exposure to dampness and the severity of asthma

Table 6 Vitamin C and FEV₁: effect modification by age and exposure to dampness

Age (yr)	Exposure to dampness in the bedroom	No of. Children^a)	FEV₁ (L/s)		Percentage increment in FEV₁ ^b)
			Placebo	Vitamin C	Mean	95% CI
7.0-8.2	Never	10	0.98	1.34	37.0%	33.9-40.2%
	During past 1 yr	11	1.04	1.34	29.0%	27.4-30.6%
	Earlier	9	0.99	1.25	27.1%	21.5-32.6%
8.3-10	Never	15	1.28	1.66	29.3%	27.6-31.1%
	During past 1 yr	9	1.21	1.52	25.9%	22.5-29.4%
	Earlier	5	1.23	1.49	21.3%	18.0-24.5%

a) Since the goal of this table is to compare the effect of "During past 1 yr" and "Earlier" exposure to dampness, one child who had both modes of exposure was excluded (age 8.4 yr), making the number of children in this analysis 59.
b) When age as a continuous variable and the three categories of dampness were included in the linear model explaining the effect of vitamin C on the FEV₁ increase, the model was improved by χ²(3 df) = 51.8, P = 10^-10 (model parameters are shown in Table 7). There was no second order interaction between age and dampness in their modification of the vitamin C effect (χ²(2 df) = 2.3, P = 0.3).

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