In this applied study, we use US county data to examine patterns in fine particulates (also called fine particulate matter or PM_2.5) ambient concentrations as a measure of air pollution within the framework of the environmental Kuznets’ curve (EKC). We pay particular attention to the role of social capital and notions of ruralness. Consistent with expectations, we find that peak of the EKC ranges between US$24,000 and US$25,500 for PM_2.5 concentrations depending on the estimator used. Also consistent with expectations, higher levels of social capital places downward pressure on PM_2.5 concentrations, but that effect is weaker in more rural areas. The implication is that the promotion of economic growth may harm the environment at lower levels of income but will improve the environment as income continues to grow.