Choking on Saharan Dust
Between November and April, Harmattan trade winds carry vast amounts of mineral dust from the Sahara Desert across West African skies toward the Gulf of Guinea. The pall of dust that hangs over the region is known as the Harmattan haze — which, fittingly, means “tears your breath apart” in Twi, a common West African language.
West Africans have long known the haze season to be one of dry skin and chapped lips, but a recent study led by Susanne Bauer of NASA’s Goddard Institute for Space Studies suggests that dusty skies are more than a nuisance. Her analysis indicates that they are deadly for hundreds of thousands of people each year.
Bauer became focused on the health impacts of dust somewhat indirectly. After completing a study in 2015 that found fertilizer use on farms was a surprisingly large source of air pollution, she wondered if there were other unexpected ways that food production was affecting air quality.
This prompted her to look closely at fires in Africa. Every year, satellites detect thousands of manmade fires that come and go in sync with the seasons. Most of these fires are ignited to clear or fertilize crops, kill pests, and manage grasslands.
Agricultural fires generate so much smoke that Bauer guessed they were one of the biggest sources of fine aerosol particles (PM2.5) — the particle size that causes the most serious health problems. (Fine particles can penetrate more easily into the human respiratory and circulatory system than larger particles.)
Bauer and colleagues tried to confirm her suspicion by running a computer simulation of how smoke, desert dust, industrial haze, and other airborne particles (aerosols) moved and evolved in African skies with changing weather and environmental conditions. The model simulated conditions in 2016, a year when researchers had ample data from satellites and from field campaigns.
To Bauer’s surprise, the analysis showed that the smoke had a smaller effect on people’s health than dust. “What we have is one of the most prolific sources of dust in the world — the Sahara Desert — regularly blowing large amounts of dust into densely populated countries in West Africa,” she explained. “When all of the dust mixes with air pollution from vehicles and factories in cities, the air becomes extremely unhealthy.” In contrast, smoke from crop fires tends to be concentrated in rural areas with relatively few people.
The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite acquired this image of dust spreading across West Africa on February 2, 2019. One of the largest sources of dust in the Sahara is the Bodélé Depression, a dried lake-bed in northern Chad that is rich with silt and fine-grained dust. The alignment of nearby mountain ranges functions like a wind tunnel, funneling strong winds over the depression on a regular basis.
By combining the results of several simulations with information about the health effects of breathing fine particles, Bauer and colleagues concluded that air pollution in Africa likely caused the premature deaths of about 780,000 people in 2016, more than the number killed by HIV/AIDS. They attributed about 70 percent of these deaths to dust, 25 percent to industrial pollution, and just 5 percent to smoke from fires. The effects of dust were especially pronounced in West African nations including Nigeria and Ghana.
“Air pollution is of overwhelming importance to public health in Africa, yet it is hardly on the radar in most countries,” said Bauer. “Except in South Africa, there are virtually no routine measurements of PM2.5; few people understand that too much exposure to air pollution can shorten lives.”
Bauer, S.E., U. Im, K. Mezuman, and C.Y. Gao, 2019: Desert dust, industrialization and agricultural fires: Health impacts of outdoor air pollution in Africa. J. Geophys. Res. Atmos., 124, no. 7, 4104-4120, doi:10.1029/2018JD029336.
NASA Earth Observatory image by Lauren Dauphin, using VIIRS data from the Suomi National Polar-orbiting Partnership. Story by Adam Voiland.