The Harvard Six Cities Study, which followed participants from 1974 to 2009, found a consistent association between chronic exposure to fine particulate matter (PM2.5) and increased mortality. The study showed that a 10 µg/m³ increase in PM2.5 was linked to a 14% increased risk of all-cause mortality, 26% for cardiovascular mortality, and 37% for lung-cancer mortality. The concentration-response relationship was linear down to PM2.5 levels of 8 µg/m³. The study also found that PM2.5 effects were stable over time, with no clear trends despite a reduction in sulfate fractions. Results from Cox and Poisson models were consistent, suggesting that reducing PM2.5 pollution could have ongoing public health benefits. The study emphasized that PM2.5 effects were not sensitive to model specifications, and that the relationship between PM2.5 and mortality was linear without a threshold. The findings support the need for public policy efforts to reduce fine particulate matter air pollution for continued public health benefits.The Harvard Six Cities Study, which followed participants from 1974 to 2009, found a consistent association between chronic exposure to fine particulate matter (PM2.5) and increased mortality. The study showed that a 10 µg/m³ increase in PM2.5 was linked to a 14% increased risk of all-cause mortality, 26% for cardiovascular mortality, and 37% for lung-cancer mortality. The concentration-response relationship was linear down to PM2.5 levels of 8 µg/m³. The study also found that PM2.5 effects were stable over time, with no clear trends despite a reduction in sulfate fractions. Results from Cox and Poisson models were consistent, suggesting that reducing PM2.5 pollution could have ongoing public health benefits. The study emphasized that PM2.5 effects were not sensitive to model specifications, and that the relationship between PM2.5 and mortality was linear without a threshold. The findings support the need for public policy efforts to reduce fine particulate matter air pollution for continued public health benefits.