The study, led by Thomas Flaherty, a researcher at the University of Birmingham in the United Kingdom, looked the effects of wood smoke, diesel exhaust, cooking fumes, and the citrusy limonene aerosols present when certain cleaning products are used. The results were reported recently by earth.com.

Fifteen healthy adults age 50 or older, all with a family history of dementia, were exposed to each of the four pollutants, and to clean air, for an hour, with two-week intervals between each of the five exposures. During the exposures, the subjects were inside a sealed chamber for 60 minutes, breathing the pollutant through a fitted mask, and the same airborne particulate count was used for each pollutant.

Cognitive and lung-function tests were performed on the subjects before each exposure and again four hours after each exposure.

Lung function dipped “subtly but measurably” after exposure to wood smoke and limonene aerosols, earth.com reported. Although the subjects’ reduced lung function was still within the healthy range, the story said, researchers had not expected to see any change.

Volunteers actually performed better on one cognition test — a reaction-time test — after breathing diesel exhaust and wood smoke than they did after breathing clean air, the earth.com story said. The study’s authors theorized that the presence of nitric oxide (NO) in those pollutants had dilated blood vessels, increasing blood flow to the brain and, thus, reaction times. Cooking emissions did not sharpen reaction times.

On a cognition test that required sustained focus rather than quick reactions, however, the volunteers performed best after breathing clean air and worst after breathing diesel fumes. “The same gases that help at one level may be hurting at another,” the earth.com story said.

Researchers also found that limonene aerosols helped subjects’ memories on the easiest version of a memory task, but cooking fumes did not.

“Each pollution source produced its own pattern of short-term changes in the lungs and the brain,” said Gordon McFiggans, professor of atmospheric science at the University of Manchester and the study’s corresponding author.

The researchers acknowledged that smells, pleasant or unpleasant, can affect alertness, mood, and attention on their own, apart from any changes their chemistry might produce in the body.

The study’s authors said they chose subjects with a family history of dementia, but no dementia diagnosis themselves, because they wanted to look at a population with a higher risk of dementia. “Investigating pollution-related short-term changes in cognitive function in this group offers early insights into how modifiable risk factors, such as pollution exposure, contribute to neurodegenerative disease trajectories,” they wrote.

It was the first clinical trial that exposed subjects to several sources of pollution with the same particulate count while measuring the effects on the lungs and brain, earth.com reported.

“There is a pressing need to respond to emerging evidence of air pollution’s impact on brain health across the life course, from early cognitive delays and adverse mental health to increased dementia risk,” the authors wrote.