People who live near busy roads have higher dementia...

"People who live near major roads have higher rates of dementia," BBC News reports.

A Canadian study found that people who lived within 50 metres of a busy road were 7% more likely to develop dementia than people who live at least 300 metres away.

The results were produced by a major study that tracked all adults in Canada's most populated province (Ontario) over 11 years.

Researchers also looked to see if a similar pattern was found with two other neurological conditions; Parkinson's disease and multiple sclerosis. They found no evidence of any link.

This study of 6.8 million people adds to evidence that living close to heavy traffic may have an effect on dementia. A study we discussed last year found evidence that particles caused by air pollution can physically make their way into human brains.

While this type of study cannot prove that traffic or air pollution has caused the increase in dementia cases, a link is certainly in the realms of scientific possibility. Air pollution caused by traffic can lead to exposure to a wide range of damaging toxins, such as nitrogen oxides.

Exactly what policy makers can do to reduce any potential risk of exposure remains a matter of debate.

On an individual basis, there's not much you can do if you live near a busy road, especially if you're in a city where most people live near busy roads. However, it does make sense to reduce your exposure to pollution if you can, for example by walking on the further side of the pavement, and exercising in parks or back streets.

Where did the story come from?

The study was carried out by researchers from a number of Canadian institutions: Public Health Ontario, Institute for Clinical Evaluative Sciences, University of Toronto, Dalhousie University, Oregon State University, Health Canada, and Harvard-Smithsonian Centre for Astrophysics in the US.

It was funded by Public Health Ontario and the Institute for Clinical Evaluative Sciences. The study was published in the peer-reviewed journal The Lancet.

The study was widely reported in the UK media on a broadly accurate basis. Most stories included warnings from independent experts that the study cannot show the cause of the increased number of dementia cases, although you have to read quite far down in most cases to see this explanation.

What kind of research was this?

This was a cohort study which tracked adults in the province of Ontario for up to 12 years. It looked at how close they lived to a main road five years before the study began, then tracked diagnoses of dementia, Parkinson's disease and multiple sclerosis.

This type of study can show links between factors such as proximity to busy roads and chances of getting illnesses, but it can't prove that one causes another.

What did the research involve?

Researchers studied health information from 6.8 million adults aged 20 to 85 in Ontario, Canada's most populous province, from 2001 to 2012.

The researchers recorded diagnoses of dementia and Parkinson's disease in people aged 55 to 85. They also recorded any diagnosis of multiple sclerosis (MS) in people aged 20 to 50 (symptoms of MS typically begin earlier than dementia and Parkinson's).

They recorded people's post code from their address in 1996, five years before the study start, and divided them into groups living within 50 metres, 50 to 100 metres, 101 to 200 metres, 201 to 300 metres, or further, from a major road.

They used information from Canadian health databases, which record diagnoses and treatments. Major roads were defined as "a major thoroughfare with medium to large traffic capacity".

They adjusted the figures to take account of the following potential confounding factors:

age and sex
pre-existing illnesses (people who already had dementia, Parkinson's disease or MS were not included in the study)
whether people lived in urban or rural areas
exposure to air pollutants using neighbourhood figures for nitrous oxide (NO2) and small particulate matter (PM2.5)

Because they didn't have information on people's individual risk factors for dementia, such as smoking, education level, physical activity and socioeconomic status, they used neighbourhood-level figures, such as average income, to estimate these individual risk factors.

They also looked at access to neurologists, which might affect people's chances of being diagnosed, and how far north or south they lived (as this has an effect on multiple sclerosis).

What were the basic results?

More than half of the 6.8 million people in the study lived within 200 metres of a major road. There were many more diagnoses of dementia than of MS or Parkinson's disease in the 12-year study:

243,611 people developed dementia
31,577 people developed Parkinson's disease
9,247 people developed MS

Researchers found no link between where people lived and how likely they were to get MS or Parkinson's disease. However, this could have been because fewer cases make it harder to get a picture of a trend.

Dementia was linked to where people lived. Compared to living more than 300 metres from a major road:

those living less than 50 metres away had a 7% increased risk (hazard ratio (HR) 1.07, 95% confidence interval (CI) 1.06 to 1.08)
those living within 50 to 100 metres had a 4% increased risk (HR 1.04, 95% CI 1.02 to 1.05)
those living within 101 metres to 200 metres had a 2% increased risk (HR 1.01 to 1.03)

Living more than 200 metres away did not increase risk. Looking at other factors, those in urban areas were more at risk. Levels of air pollution (NO2 and PM2.5) explained some of the increased risk associated with living close to a busy road, but not all of it.

How did the researchers interpret the results?

The researchers said the study provides "important insights into a possible role of near-road exposure on the development of dementia". They say that even though a 7% increase in risk is small, because of the numbers of people getting dementia and the numbers living in towns and cities, "even a modest effect from near-road exposure can pose an enormous health burden."

Conclusion

Dementia is a growing problem as more people live longer. We don't yet know exactly how it develops, and it seems likely that a number of factors affect a person's chances of getting it, including genetics, lifestyle and other environmental factors.

This study seems to add to evidence that something about living near busy roads – whether it be air pollution, noise or other unknown factors – also has an effect on the chances of getting dementia. There are several limitations to be aware of, however:

the study only looked at where people lived at one point in time, and we don't know how well that represents their exposure to road noise or air pollution over the 12 year study period
we don't know how people's individual behavioural risks might have affected the results. For example, people living near busy roads might take less physical exercise than those living in quieter areas
some people with dementia or other diseases might not have been diagnosed

Overall, this is a very large study which adds to concerns over pollution and health. Governments and health authorities should be aware of this research when putting together plans to tackle air pollution and when planning roads and housing.

Although nothing guarantees that you won't develop dementia, there are plenty of things you can do that may help delay the onset of the condition: