LAGOS, Nigeria – In fast growing countries, air pollution generally increases sharply as their population and economies grow. However, a new study on air quality in Africa published on Monday found the opposite: one of the most dynamic regions of the continent is less polluted.

The study, published in Proceedings of the National Academy of Sciences, found that levels of dangerous nitrogen oxides, a by-product of combustion, have fallen sharply in northern sub-Saharan Africa as the region’s wealth and population have increased.

“The traditional paradigm is that as middle and low-income countries grow, there are often more emissions and it is very interesting to see a different type of trajectory,” said Jonathan Hickman, a researcher at NASA’s Goddard Institute for Space Studies Lead author of the study. “It’s nice to see a decline when you expect pollution to increase.”

The reason for this, according to researchers, is that increases in industrial and traffic pollution in the area under study – from Senegal and Ivory Coast in the west to South Sudan, Uganda and Kenya in the east – appear to have been offset by a decrease in the number of fires caused by farmers.

While Africa is not a major industrial polluter like Asia and North America, biomass has long been burned frequently in the dry season.

Burning vegetation is believed to be a cheap and efficient way to clear land in preparation for the planting season, and burning has the advantage of preserving mineral nutrients in the soil. However, the consequences for human health and global warming can be severe. Land management fires can be combined with urban pollution to create toxic air. And fires release planet-warming carbon dioxide into the atmosphere.

Brush fires tend to conjure up images of runaway flames in places like Australia or the western United States, but North Equatorial Africa is the region with the largest area of ​​biomass burned land burned at around 70 percent of the world, according to researchers.

The new study used NASA satellite data and imagery to measure hazardous gases in the area’s air and determine fire trends between 2005, when NASA records began, and 2017. At the height of the fire season, nitrogen dioxide or NO2 levels are harmful.The gas produced by road traffic and other fossil fuel burning, which has been linked to breathing problems such as asthma, decreased 4.5 percent in the lower atmosphere.

That decline was so significant, said Dr. Hickman that it resulted in a net loss of the pollutant in the region.

The result is important as Africa’s growing population, currently 1.2 billion but projected to exceed 2 billion by 2040, is rapidly urbanizing. Pollution has surpassed AIDS as the leading killer on the continent. However, governments often prioritize economic growth over the environment, which means little emphasis is placed on collecting air quality data or establishing clean air policies.

The new study “provides an important tool to fill some of these data gaps in Africa, where air pollution studies are lacking on multiple levels,” said Andriannah Mbandi, a Kenya-based environmental researcher affiliated with the Stockholm Environment Institute. “It would be great if the follow-up to this paper quantified these values ​​using health and economic metrics, which is useful for policy makers.”

While the fires can recede, pollution continues to increase.

Emissions from fossil fuel burning are expected to increase significantly in Africa. Despite the African Union’s commitment to green energy in 2015, 80 percent of the electricity generated on the continent comes from coal or other fossil fuels. More and more used cars are being imported, which increases emissions from transport.

This could trigger a reversal of the positive trend found in the study on Monday, especially in populous, richer countries like Nigeria.

“When you increase GDP, you see a decrease in the amount of NO2, but it only followed that pattern up to a point,” said Dr. Hickman used the analysis the team had done and tinkered with wealth and pollution within the model.

“At the highest levels on this GDP metric, air pollution was almost back to where it was when it started,” he said. “What is suggesting is that this decline that we are seeing is likely to slow and possibly reverse as a result of increased fossil use.”