Another challenge in quantifying the health effects of multi-pollutant exposure is how to estimate the risk of adverse health responses from multiple pollutants. As stated above, single pollutant approaches and even EWAS in which the unit of analysis is based on a single pollutant have had small to modest effect sizes. The challenge is to construct the disease risk from exposure to multiple environmental risk factors [16]–[18]. Some advances have been made in the air pollution area (air pollution mixtures). For example, in the indicator approach one pollutant represents the combined exposure to several pollutants [19], [20]; or, in the source apportionment approach particle constituents are assigned to emission sources using principal component analysis and hierarchical clustering [21], [22]. However, these approaches do not account for a wide range of environmental pollutants.
