Pensioners should eat plenty of fish to help reduce the amount their brain shrinks as a result of air pollution, a study has suggested.
Omega-3 fats in oily fish such as salmon, sardines and mackerel reduce inflammation of the brain and help it retain its structure, according to scientists.
This makes it a key ingredient in battling the toxic effects of fumes from transport and industry, researchers from Columbia University in New York concluded.
Air pollution has been at the centre of many studies examining risk factors for cognitive impairment and the likelihood of developing dementia. Alzheimer's Society said there is a "strong case for further research into the effect of air pollution on brain health".
In this study, researchers examined the brains of 1,315 healthy women with an average age of 70 and asked them to complete questionnaires about their diet, physical activity and medical history.
They also calculated how much oily fish they were eating on average every week, and their three-year average exposure to air pollution.
Those who had higher levels of omega-3 in their blood had more white matter and their hippocampus was greater in volume, according to MRI scans.
Dr Ka Kahe, who led the study, said: "Our findings suggest higher levels of omega-3 fatty acids in the blood from fish consumption may preserve brain volume as women age and possibly protect against the potential toxic effects of air pollution.
"It's important to note our study only found an association between brain volume and eating fish.
"It does not prove that eating fish preserves brain volume. And since separate studies have found some species of fish may contain environmental toxins, it's important to talk to a doctor about what types of fish to eat before adding more fish to your diet."
He added: "Fish are an excellent source of omega-3 fatty acids and easy to add to the diet.
"Omega-3 fatty acids have been shown to fight inflammation and maintain brain structure in ageing brains. They have also been found to reduce brain damage caused by neurotoxins like lead and mercury.
"So we explored if omega-3 fatty acids have a protective effect against another neurotoxin - the fine particulate matter found in air pollution."
The findings were published in the journal Neurology.