Table 4– Combined effect estimates of particulate matter with a 50% cut-off aerodynamic diameter of 10 μm (PM₁₀) on respiratory symptoms from single- and two-pollutant models

	Studies n	Mean OR (95% CI)^#
	Studies n	Single-pollutant model	Two-pollutant model
Bronchitis
In studies with SO₂ data	3	1.17 (0.93–1.48)	0.81 (0.44–1.51)^¶
In studies with NO₂data	3	1.35 (0.96–1.91)	1.49 (0.85–2.61)
Phlegm
In studies with SO₂ data	2	1.23 (0.81–1.89)^¶	1.24 (0.80–1.94)^¶
In studies with NO₂data	2	1.55 (1.11–2.18)	1.55 (1.01–2.38)
Nocturnal cough
In studies with SO₂ data	2	1.41 (0.68–2.93)^¶	1.43 (0.61–3.36)^¶
In studies with NO₂data	3	1.14 (0.68–1.91)^¶	0.79 (0.53–1.19)
Morning cough
In studies with SO₂ data	5	1.25 (0.93–1.67)^¶	1.24 (0.90–1.70)^¶
In studies with NO₂data	3	1.42 (1.04–1.92)	1.36 (0.87–2.13)
Hay fever
In studies with SO₂ data	5	1.13 (0.98–1.31)	1.19 (1.01–1.40)
In studies with NO₂data	4	1.22 (0.84–1.78)^¶	1.18 (0.70–1.97)
Itchy rash
In studies with SO₂ data	3	1.05(0.93–1.19)	1.13 (0.97–1.33)
In studies with NO₂data	3	1.18 (0.94–1.50)	0.91 (0.48–1.71)^¶
Allergy to pets
In studies with SO₂ data	3	1.31 (0.90–1.90)^¶	1.16 (0.88–1.52)
In studies with NO₂data	3	1.25 (0.92–1.70)	0.87 (0.42–1.80)^¶

Combined effect estimates calculated from country-specific estimates using random effects model. ^#: adjusted for age, sex, maternal education, paternal education, household crowding, current parental smoking, mother smoking during pregnancy, gas cooking, unvented gas/oil/kerosene heater, mould, nationality, birth order, and “ever had a pet” and expressed per 10-μg·m⁻³ increase in PM₁₀;^¶: evidence of between study heterogeneity (p<0.10).