“We were looking at ozone and particulate matter, the two biggest air quality issues in the United States,” Tammy Thompson, a research scientist at Colorado State University with the Cooperative Institute for Research in the Environment, told me.

Reasonable people disagree over the economic effects of cap and trade, the politically divisive market-oriented system designed to limit air pollutants with incentives that reward efficiency and innovation. But Thompson, along with Noelle Selin, Sebastian Rausch, and Rebecca Saari at MIT write that any cost-benefit analysis of climate policy that omits the health effects of regional air pollution “greatly underestimate[s] benefits.”

There are national air quality standards that cover ground-level ozone and particulate matter, but this study showed that policies aimed at reducing carbon emissions could improve air quality in amounts comparable to those specifically targeting air pollution. “When you put a constraint on carbon emissions in the economy,” Thompson explained, “with that comes co-benefits, where you’re also reducing nitrogen oxides, sulfur dioxides, and other pollutants that react to form ozone and particulate matter that can cause human health issues.”

As carbon emissions are reduced, so are nitrogen oxide and sulfur dioxide, and therefore ozone and particulate matter. So this research model estimated how the latter would be reduced under various carbon policies. In addition to an economy-wide cap and trade, the researchers also looked at narrower cap-and-trade regulations that would focus on the energy and transportation sectors individually, which are known for human health-associated air pollution.

In June, the Environmental Protection Agency proposed the Clean Power Plan, which the agency claims will prevent 2,700 to 6,600 premature deaths in 2030 alone, as well as 140,000 to 150,000 asthma attacks in children. It imposes carbon emission caps that focus on the electricity sector, a model that Thompson and company found less economically efficient than a broader limit on carbon emissions for the entire economy. Thompson said that’s not a surprise; the more narrow and stringent the policy, the more it costs to implement. “I think economists have been screaming this for decades: Let the market decide.”

“We were really interested in looking at how the health response would change if we targeted these sectors that were known to cause air pollution," Thompson said. "We were surprised to see that the human health benefits did not change very much from scenario to scenario. The cost-benefit ratio was largely driven by the cost of the policy, and the economy-wide policy had fewer economic constraints, so that was the cheapest option.”

Lest this research, which was supported by EPA funding, seem like a clear call for economy-wide cap and trade, Thompson pushed back at my suggestion that she had solved the puzzle.