Often, when populations and economies boom, so does air pollution—a product of increased fossil-fuel consumption by vehicles, industry and households. This has been true across much of Africa, where air pollution recently surpassed AIDS as the leading cause of premature death. But researchers have discovered at least a temporary bright spot: dangerous nitrogen oxides, byproducts of combustion, are declining across the north equatorial part of the continent. The reason: a decline in the longtime practice of setting of dry-season fires to manage land.