Q&A: Does air pollution increase the risk of dementia?

In a new Burden of Proof Response Function analysis published in Nature Aging, IHME researchers performed a meta-regression for the all types of dementia combined (plus AD and vascular dementia separately — two major subtypes of dementia) to understand the relationship between fine particulate air pollution and the risk of developing these diseases. We speak with Dr. Katrin Burkart, co-author and Assistant Professor at IHME on new findings uncovered in this analysis.

This transcript has been lightly edited for clarity

What prompted the research into a possible link between particulate air pollution and increased risk of dementia?

As dementia disease increases, there’s also an increased interest in understanding what are the risk factors, other than age, that aggravate dementia disease. And air pollution has been one of these risk factors that has been highlighted by scientists. There’s actually a fair number of epidemiological studies that do support a relationship, meaning we do observe higher incidence or risk in population groups that are exposed to higher levels of particulate matter.

This is especially disconcerting as about 90% of the population are exposed to air pollution levels that are above the WHO guidelines. And in the study we just published, we systematically assessed and evaluated all the evidence out there. And we did this through the lens of the newly developed Burden of Proof framework. The strength of this framework is that it allows us to incorporate all the available evidence.

It allows us to adjust for study-specific characteristics and in-between study heterogeneity. It also allows us to estimate a non-linear exposure–response relationship, and finally, it also allows us to translate the strength of the evidence into a star rating that can be intuitively understood by the lay public and by policymakers.

What were the key findings of this research?

The most important finding certainly is that we did observe a robust and significant relationship between particulate matter and dementia. When we included all types of dementia, we found a two-star rating. But when we only looked at Alzheimer’s disease, the strength of evidence actually increased, and we observed a three-star rating. On the other hand, for vascular dementia, we did not find a relationship with particulate matter. And this might be either due to limited data, or a lack of data quality, or substantial heterogeneity between the different studies.

What factors are critical to producing a risk–outcome score for particulate matter pollution and dementia?

There are two aspects that fundamentally do determine the risk–outcome score. And these are the magnitude of the risk increase, and the differences in the risk increase between studies.

For air pollution, actually, the relative risk increase isn’t too high, and the public health relevance really lies in everyone being exposed all the time. What is typical for air pollution studies is that we find a lot of in-between study heterogeneity, and this is possibly due to challenges in exposure and outcome assessment, as well as differences in study designs.

So, both the rather small and moderate relative risk increase, and the in-between study heterogeneity put us somewhere in the middle of the evidence range, which is not uncommon for air pollution studies.

When we drill down in our studies and only include studies with older adults, about the age of 60, or we only include studies that use active dementia ascertainment, we do find stronger evidence, with a three-star rating, which really indicates that when we reduce study differences or study diversity, we find stronger evidence.

Generally, the impact of air pollution on dementia is a really newly emerging field, and we do need more research that so that we can be more confident about our scoring.