Epidemiological studies usually report the associations between one or more pollutants and health. However, pollutants such as PM, NO2 (nitrogen dioxide) and CO are often strongly correlated and occur as components of the complex urban air pollution mix. This correlation makes it difficult to accurately determine the independent effects of specific pollutants. 1 Common ambient air pollutants have similar mechanisms and consequences on human health, further complicating the process of allocating the precise role each individual pollutant plays (neutral, additional or synergistic). Therefore, simply summing the specific impact of correlated individual pollutants (as reported by epidemiological studies) could lead to double.
