There is strong evidence that fine particulate matter (aerodynamic diameter <2.5 μm; PM_2.5) air pollution contributes to increased risk of disease and death. Estimates of the burden of disease attributable to PM_2.5 pollution and benefits of reducing pollution are dependent upon the shape of the concentration response (C-R) functions. Recent evidence suggests that the C-R function between PM_2.5 air pollution and mortality risk may be supralinear across wide ranges of exposure.