Cerza, F., Renzi, M., Gariazzo, C. et al. Long-term exposure to air pollution and hospitalization for dementia in the Rome longitudinal study. Environ Health 18, 72 (2019) https://doi.org/10.1186/s12940-019-0511-5
No one doubts that breathing is essential for human life. But when the air you inhale is tainted with pollutants, that life-giving breath could have unintended, negative consequences for your health later in life. A new study explores whether there might be an association between long-term exposure to air pollution and the development of dementia.
How might air pollution impact risk of dementia?
Before we dive into the research, let’s pause and assess the plausibility of the hypothesis. We can safely assume that the researchers suspect that there might be a cause-and-effect relationship at play between air pollution exposure and dementia. By testing whether there is a statistical association between these two factors, they’re trying to establish one piece of the puzzle: if higher levels of air pollution exposure are tied to a greater number of hospitalizations for dementia, it strengthens the argument that the two factors are related.
One thing that strengthens a case for cause-and-effect is biological plausibility: what is the mechanism by which exposure to air pollutants might trigger or increase the likelihood of dementia? Many other scientists have explored this question already and no one has a clear answer yet. However, some researchers are intrigued by the inflammatory response that air pollutants, like particulate matter, appear to trigger in mammals. That same inflammatory response is being explored as a possible cause of (or risk factor for) the development of dementia. So, it appears theoretically plausible that air pollution might increase risk of dementia, via the inflammatory response. With this in mind, we can approach the question of association with a bit more confidence.
How did this research look for possible associations between air pollution and dementia?
These researchers used data collected in a project called the Rome Longitudinal Study. Residents of Rome, Italy who were part of the 2001 census were asked to complete a questionnaire and then were followed over time using administrative records (such as hospital and death records). In this study, researchers whittled down the participants from the Rome Longitudinal Study to look only at those residents who were 65 years of age or older in 2001. The study followed 350,844 senior residents until 2013.
Using the addresses of the residents in 2001, researchers attempted to estimate a person’s level of exposure to air pollutants like particulate matter, ozone, and nitrogen oxide. Another Roman study (the European Study of Cohorts for Air Pollution Effects) had measured these pollutant levels across the city in three two-week periods in 2010. They also estimated distance to the nearest high-traffic road (defined as >10,000 vehicles per day) from the address, to try to measure exposure to traffic. Using this information on spatial estimates of exposure to pollutants based on residential addresses, researchers could estimate the participants’ air pollution exposure.
The researchers also used hospital discharge records from 2001-2013 to identify cases of dementia-related hospitalizations. They wanted to look at first hospitalizations for dementia, and so excluded participants who had been hospitalized for dementia before 2001 (this equated to 1% of the subjects). Where possible, they recorded the specific subtypes of dementia. Alzheimer’s disease is probably the most well-known type of dementia; it’s also the most common (accounting for an estimated 60—80% of cases). The second most common type is vascular dementia and the category of ‘senile dementia’ is often used when a more specific diagnosis is elusive.
The average age of the residents in 2001 was 74.5 years. Over the ten years of the study, about 6% of the residents would be hospitalized for the first time with dementia.
What did the analyses show?
Results of the analyses were mixed. While increased exposure to ozone and nitrogen oxide were associated with an increased number of hospitalizations for dementia (as the researchers expected), increased exposure to nitrogen dioxide was instead associated with a decreased number of hospitalizations. Total air pollution also did not paint a straight forward picture; results showed that greater exposure was strongly associated with an increase in hospitalizations for vascular dementia, but not for Alzheimer’s or senile dementia.
This latter finding aligns well with other research demonstrating a clear relationship between exposure to air pollutants and cardiovascular disease; air pollution could be a factor in the impairment of blood flow to the brain which defines vascular dementia.
What are the limitations of the research?
There are handful of limitations to this study to consider. First, it only looks at hospitalizations for dementia, which is a bit indistinct. Someone suffering only minor cognitive impairment may not visit a hospital, so the study may be undercounting the true number of dementia cases.
The study was also restricted by its use of limited census data. The research team was not able to assess the potential impact of important risk factors like smoking status, body mass index (BMI), and genetic factors, as these data were not available, but are known to impact health status.
Further, the time points in the data used are a bit disjointed. The residential address of the participants in 2001 was used to estimate their exposure to pollutants, and the pollutant levels in those areas were only measured in 2010. We can ask a number of questions here: What if the participant moved between 2001 and 2010? What if their residential address is not where they spend most of their time (and get most of their exposure)? What if the exposure residents experienced at work is more impactful than exposure at home? What if they lived somewhere else before 2001 and exposure earlier in life is more associated with dementia risk than later in life?
The researchers conclude that their findings do demonstrate some possible associations between certain types of air pollution and the development of certain types of dementia. But as the old adage goes, ‘correlation does not equal causation’. Much more evidence is needed before a cause-and-effect relationship can be considered likely. Given the limitations of the research described above, the results should be interpreted cautiously.