As the incidence rates of dementia climb across the globe, the need to identify modifiable risk factors, preventive measures, and early treatments has become a public health priority. While some modifiable risk factors have been identified, there is still a considerable amount of unexplained risk. There is some evidence that environmental factors may account for some of this unexplained risk. The researchers in this study reviewed 60 studies to identify the leading environmental risk factors associated with dementia.1
The researchers determined that all types of air pollution, aluminum, vitamin D, and electric and magnetic fields have a relationship to the development of dementia. Identifying and then addressing modifiable environmental risk factors for dementia could have a profound impact on the future costs of dementia and the lives of those who are at risk for and suffer from dementia.
Studies that did not have dementia as an outcome measure were excluded from the review of literature.1 The researchers noted this was a limitation of the study, as these types of studies may have shed light on the relationship between environmental factors and cognition or brain function. Also, studies that measured brain or serum levels of a specific substance were excluded from the review because the physiological changes could not be linked directly to an environmental exposure. The researchers also excluded individual case studies, animal studies, and nutritional studies. The researchers also noted that an inherent bias present was that the majority of the research was conducted in high-income countries, while the projected increase in dementia rates is for a disproportionate increase in low-to-middle income countries.2 The researchers identified the short-comings of their literature review to include cross-sectional, rather than longitudinal studies, lack of consistency of measures between studies, and insufficient numbers to demonstrate a true causal relationship.1
A study of almost 6,000 people across five provinces in China found higher levels of environmental tobacco smoke to be associated with increased risk for severe dementia.3 A Taiwanese study following nearly one million people over a ten year period found an association between baseline ozone exposure and Alzheimer’s dementia.4 Also, at follow-up they found that a change in exposure to ozone and particulate matter up to 2.5 μm in diameter was associated with a two-to-three-fold increased risk of Alzheimer’s.4
Toxic Heavy Metals
A small case–control study (n=129 people in each group) identified that a surplus of people with Alzheimer’s were born in areas with higher than average lead concentrations.5 However, overall there was very little evidence for an association between toxic heavy metal exposure and dementia.
Aluminum was the most studied metal, with sixteen studies evaluating the relationship between aluminum and dementia. These studies accounted for nearly 22,000 people with dementia. Specifically, a study of nearly 4,000 older adults in France found that aluminum levels in drinking water in excess of 0.1mg per day were associated with a doubling of dementia risk and a three-fold risk for Alzheimer’s.6,7 While the larger studies on aluminum showed an association between aluminum and dementia, the smaller studies showed a null effect.
There were studies that looked at manganese, molybdenum, nickel, uranium, zinc, copper, iron, calcium, and cobalt. However, these studies were small and their evidence tended to be weak or the studies were contradictory. Aluminum was the main metal that had several large studies that supported the association between its presence and risk for dementia.
Other Trace Elements
One small study (n=160) found that areas with the highest concentrations of fluoride in the public drinking water had the lowest incidence of dementia.8 There have been multiple studies evaluating selenium, with mixed reviews as a protective element at this time. Likewise, the information on silicon in drinking water remains contradictory and inconclusive.
Occupational Related Exposures
An occupational related exposure study of 1552 people with dementia found mixed evidence for extremely low frequency magnetic fields or electric shocks.9 The other study results evaluating the effects of occupational related exposures to dementia are weak. The evidence for solvents is mixed. There is stronger evidence for pesticides, but the findings in this literature are mixed as well. The researchers in this study could not find any published evidence suggesting occupational exposure to lead, diesel motor exhaust, ins, dyes, paints, stains, solvents, gasoline, oil, liquid plastics, rubbers, or vibratory tools is associated with dementia risk.
Miscellaneous Environmental Factors
Two different reviews that examined low and extremely low frequency electric and magnetic fields found mixed evidence, but the researchers determined that there seems to be an association with dementia.9,10 A study in Switzerland found that living close to power lines for over 15 years was associated with a doubling of Alzheimer’s disease mortality, which supported the researchers’ previous determination of an association between electric and magnetic fields and dementia risk.11
Three large studies (n=11,884, with 691 developing dementia) found that having a lower vitamin D level at baseline was associated with an increased risk of developing dementia.12, 13, 14
The researchers found evidence for dementia risk being linked to air pollution exposures, especially nitrogen oxides, particulate matter, and ozone. Aluminum was the only metal the researchers found that may influence dementia risk. Silicon was the only trace element identified as possibly affecting dementia risk, although it and selenium require further investigation. Occupational exposure to some pesticides and metals may affect dementia risk. The researchers identified moderate evidence for electromagnetic fields, though they identified that this exposure is complicated and requires further detailed analysis. Finally, the researchers found strong evidence that vitamin D deficiency was associated with increased dementia risk.
Dementia is affected by protective factors and risk elements occurring throughout one’s life. As such, identifying the moment in the life course that the exposure is measured is important. Epidemiological evaluations of risk include evaluating accumulation of risk, sensitive periods when an exposure has a greater effect than at other times, and critical periods that are an isolated moment in time when an exposure has an effect.15,16 For example, the researchers identified an association between air pollution and dementia risk, but the study did not identify what stage of life exposure was most important.1 As researchers are beginning to unravel the life course of the development of dementia, identifying which life stage for exposure risk is important.
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