We already know air pollution can be bad for our lungs.
But a new study shows dirty air can also damage our brains.
Researchers at the International Food and Policy Research Institute examined data from a Chinese study that mapped the cognitive test scores of nearly 32,000 people between 2010 and 2014 against their exposure to short- and long-term air pollution.
The findings showed the more a person was exposed to air pollution, the lower their verbal and math test scores were.
And the decline was worse among older, less educated men.
Pollution could also be a major risk factor for developing dementia and Alzheimer's disease.
The World Health Organization estimates 90 percent of the world's population breathes highly polluted air, with Africa and Asia being the most affected regions.
The most polluted cities were found in developing countries.