Pre-industrial and recent data series were used to assess the evolution of ozone concentrations in northern Portugal, being the results analysed according to the atmospheric chemistry. Compared to the nineteenth century, current concentrations were in average 147% higher due to the increased photochemical production, associated with the increased anthropogenic emissions. The models developed were considered valuable tools for public health protection because they could make efficient predictions of ozone levels, thus allowing to provide early warnings to the population. The impact of ozone concentrations on the prevalence of childhood asthma was assessed eliminating confounding effects by the assessment of the influence of ozone on biological pollutants and by selecting an area to perform the study with high ozone concentrations and low of other pollutants (exposed area). Children living at the exposed area had a 3 times higher risk (relative risk 2.84; odds ratio 3.02) of having asthma than those living at the unexposed area.