Pooled relative risks (and associated 95% CI) and heterogeneity measures (I2 and Q p-value) from random-effects models for the association between respiratory admissions following a 10 000 particles·cm−3 increase in lag 3 particle number concentration (PCN) by city characteristics
| Characteristic | Cities n | Relative risk (95% CI) | I2 % (Q p-value) | p-value for between groups difference |
| Western European cities only | 8 | 1.00 (0.99–1.00) | 58 (0.028) | Not applicable |
| Exposure from background sites | 10 | 1.01 (0.99–1.02) | 21 (0.249) | Not applicable |
| Upper limit of particle size | ||||
| <100 nm | 8 | 1.00 (0.97–1.03) | 2 (0.417) | 0.519 |
| >100 nm | 3 | 1.01 (0.98–1.04) | 75 (0.017) | |
| By mean daily levels of PNC# | ||||
| <6000 particles·cm−3 | 6 (Augsburg, Copenhagen, Chernivtsi, Dresden, Ljubljana, Prague) | 1.03 (1.00–1.05) | 0 (0.916) | 0.035 |
| >6000 particles·cm−3 | 5 (Beijing, Barcelona, Helsinki, Rome, Stockholm) | 1.00 (0.98–1.02) | 66 (0.020) | |
| By mean daily levels of PNC¶ | ||||
| <10 000 particles·cm−3 | 8 | 1.01 (0.99–1.03) | 7 (0.375) | 0.604 |
| >10 000 particles·cm−3 | 3 | 1.00 (0.96–1.05) | 74 (0.020) | |
| Climate | ||||
| Cold | 7 | 1.01 (0.99–1.03) | 19 (0.281) | 0.608 |
| Hot | 4 | 1.00 (0.97–1.03) | 62 (0.048) |
#: below or above the median of the contributing cities’ levels; ¶: for comparability with previous results.