Could where you live give you Alzheimer's?

Even during the holidays, main roads around Auckland see a constant flow of cars and trucks passing by homes.

Traffic pollution is known to cause a range of health problems, from heart disease to cancers. Now, Canadian scientists say it could be linked to dementia.

"Our study, along with a much broader research literature, does provide a pretty good public health argument for further measures to reduce emissions from traffic," says Dr Ray Copes of Public Health Ontario.

"I think it also raises the question of how we should plan and lay out our communities."

The study of more than 6 million Ontario residents found 243,000 cases of dementia over 12 years, then mapped their proximity to major roads.

The closer people lived to a busy road, the higher the risk.

At 50 metres the risk was 7 to 11 percent higher, at 50 to100 metres the risk was up 4 percent, at 101 to 200 metres it fell to 2 percent, and at more than 200 metres there was no elevated risk.

But the causes of dementia are still largely unknown, and there are many possible factors - so people living near busy roads shouldn't be unduly worried.

"I think this is the first study showing this link, so for now people living near a busy road shouldn't be concerned, but it does warrant further investigation," says Dr Clare Walton of the UK Alzheimer's Society.

"But what we do know is that smoking, a poor diet and a sedentary lifestyle also increase your risk, and they're things that people can take action on now."

The authors of the study admit it doesn't prove that heavy traffic causes dementia, but it certainly adds to the growing body of evidence that air pollution is bad for our health.