Data from 337 cities across 18 countries show that even slight increases in ambient carbon monoxide levels from automobiles and other sources are associated with increased mortality. Researchers analyzed data, including a total of 40 million deaths from 1979 to 2016, and ran it through a statistical model. The research in The Lancet Planetary Health also shows that even short-term exposure to ambient carbon monoxide (CO)—at levels below the current air quality guidelines and considered safe—had an association with increased mortality. Overall, a 1 mg/m³ increase in the average CO concentration of the previous day was associated with a 0.91% increase in daily total mortality, the study finds. This suggests reducing ambient CO concentrations through stricter control of traffic emissions and other measures could achieve considerable public health benefits.