Table 2

Pooled effect estimates for peak expiratory flow (PEF), where n represents the number of studies and p values derive from the Q combinality test for homogeneity between study effect estimates

Pollutant	Method of pooling or subgroup	Change in PEF (l.min⁻¹) per unit (μg.m⁻³) increase in pollutant			Expected impact on PEF (l.min⁻¹) of a 50 μg.m⁻³ rise in pollutant	Q test p value	n
Pollutant	Method of pooling or subgroup	Pooled effect estimate	95% confidence interval			Q test p value	n
*Number greater than the total number of studies as some studies present results separately for symptomatic and non-symptomatic/asthmatic children.
PM_2.5	Overall pooled effect estimate
	Fixed effects model	−0.063	−0.091	−0.034	−3.15	<0.05	5
	Random effects model	−0.144	−0.243	−0.044	−7.20
PM₁₀	Overall pooled effect estimate
	Fixed effects model	−0.012	−0.017	−0.008	−0.60	<0.001	13
	Random effects model	−0.033	−0.047	−0.019	−1.65		13
	Subgroups based on the population studied (fixed effects models)
	Asthmatic or symptomatic children	−0.010	−0.014	−0.005	−0.50	<0.005	9*
	Non-symptomatic children	−0.039	−0.055	−0.024	−1.95	<0.001	5*
	Subgroups based on environmental conditions (fixed effects models)
	High ozone levels	−0.041	−0.074	−0.009	−2.05	<0.025	3
	High SO₂ levels	−0.025	−0.047	−0.003	−1.25	−	1
	High PM₁₀ levels only	−0.011	−0.016	−0.007	−0.55	<0.001	9
	Subgroups based on pollutant lag (fixed effects models)
	Same day or prior day only	−0.009	−0.014	−0.005	−0.45	<0.001	8
	Subgroups based on analytical approach fixed effects models)
	Use of a two stage approach	−0.033	−0.054	−0.013	−1.65	<0.025	3
	Use of a population daily average outcome	−0.011	−0.016	−0.007	−0.55	<0.001	10