IR are presented as %. Chronic obstructive pulmonary disease (COPD) was identified as principal diagnosis of COPD or respiratory failure with COPD as secondary diagnosis; lower respiratory tract infections (LRTI) (bronchitis and pneumonia) were identified as principal diagnosis with COPD as secondary diagnosis. PM₁₀: particles with a 50% cut-off aerodynamic diameter of 10 μm. ^#: risk estimated at the best lag, hospitalisations for all respiratory diseases was lag 0–1 days, for COPD was lag 0 days, for LRTI in COPD was lag 0–3 days, and out-of-hospital mortality was lag 0–3 days; ^¶: p-value of heterogeneity test (null hypothesis is perfect homogeneity of city-specific results); ⁺: p-value derived from the model with the interaction term (for each condition, the reference category is the group of subjects without the disease);^§: discharge diagnosis in the period 29 days to 2 years before death, for International Classification of Diseases (ninth revision) codes 410–414. Bold type indicates the total effects, the statistically significant results within each variable, and the p-interaction values that identify an effect modification.