As main markers for pollution were used two types of markers: simple markers, represented by the HMs concentrations in contaminated soils and by the HMs concentrations in the vegetables consumed by the population in these areas.
The use of NO2 as a marker for traffic pollution may, however, hide complex chemical processes that occur in the atmosphere.
Seasonal variation also was observed for O3, a marker for regional pollution, which exhibited positive correlations with daily ambient PM2.5 during the summer (r = 0.2) but inverse correlations during the spring (r = −0.7).
Because NO(2) can be considered a marker for air pollution from traffic, the more easily measured variables degree of urbanization, traffic density, and distance to a nearby highway can all be used to estimate exposure to traffic-related air pollution.
In the present study we used three markers for air pollution from traffic at the residences, which were moderately correlated (r between 0.43 and 0.53).
Finally, the air pollution mixture on high ozone days in London may be different from the air pollution mixture on low ozone days; if so, high ozone concentrations may be acting as a marker for other pollutants (or a mixture of pollutants) that might be responsible for the observed health effects.
In the Fresno/Clovis study area, the main source for NO2 is traffic; therefore, NO2 is a reasonable marker for traffic-related pollution (Watson et al. 2000).
The purpose of the present study was to determine whether the incidence of postmenopausal breast cancer in Montreal, Quebec, was associated with exposure to intraurban concentrations of nitrogen dioxide (NO2), a marker for traffic-related pollution.
Therefore, the extent to which the observed associations are related to a direct effect of NO2 and/or reflect the fact that NO2 is a marker for the urban pollution mix, particularly for ultrafine particles PM (Seaton and Dennekamp 2003), remains to be investigated.
