Race or racial segregation? Modification of the PM_2.5 and cardiovascular mortality association

Background Many studies have identified an inequitable distribution of exposure to PM_2.5 (particulate matter less than 2.5 microns) by race. We investigated the association of PM_2.5 and cardiovascular mortality considering both the decedents’ race and neighborhood racial composition as potential modifiers. Methods We obtained geocoded cardiovascular mortality records of all black and white decedents from urban block-groups in Massachusetts between 2001 and 2011 (n = 130,863). We examined the association between PM_2.5 and cardiovascular mortality, and assessed effect modification by three types of racial modifiers: decedents’ race, census block-group percent black and white, and two novel measures of racial segregation. The Racial Residential Segregation (RRS) quantifies the concentration of non-Hispanic blacks and whites in each block-group. The Index of Racial Dissimilarity measures dissimilarity in non-Hispanic black and white racial distribution between the smaller census block-group and larger tract. Results We found a 2.35%(95%CI: 0.92%;3.79%) increase in mortality for each 10μg/m³ increase in two-day average exposure to PM_2.5. The effect was modified by the block-group racial composition, with higher risks in block-groups with the highest percentage of black residents (interaction p-value = 0.04), and in block-groups with the lowest RRS (i.e. higher black to white resident ratio, interaction p-value = 0.072). Racial dissimilarity did not modify the associations. Conclusion Current levels of PM_2.5 are associated with increased cardiovascular deaths in Massachusetts, with different risks between areas with different racial composition and segregation. This suggests that pollution reductions in neighborhoods with the highest percentage of non-Hispanic blacks would be most beneficial in reducing cardiovascular mortality and disparities.