The latter could increase one’s risk of being exposed to substances that are toxic to the nervous system such as air pollution,151 lead152 and pesticides.153 In addition, people with fewer years of education tend to have more cardiovascular risk factors for Alzheimer’s, including being less physically active,154 having a higher risk of diabetes,155-157 and being more likely to have hypertension158 and to smoke.159 Social and Cognitive Engagement Additional studies suggest that remaining socially and mentally active throughout life may support brain health and possibly reduce the risk of Alzheimer’s and other dementias.121,160-172 Socially and cognitively stimulating activity might help build cognitive reserve. However, it is also possible that undetected cognitive impairment decreases one’s interest in and ability to participate in activities involving social and cognitive skills. In this case, the association may reflect the effect of cognitive impairment on social and cognitive engagement rather than the effect of engagement on dementia risk. More research is needed to better understand the biological processes that link social and cognitive engagement to dementia risk. Traumatic Brain Injury (TBI) TBI is the disruption of normal brain function caused by a blow or jolt to the head or penetration of the skull by a foreign object. TBI increases the risk of dementia.173 According to the Centers for Disease Control and Prevention (CDC), nearly 3 million TBI-related emergency department visits, hospitalizations and deaths occurred in 2014, the latest year for which information is available.174 The leading causes of TBI that resulted in emergency department visits were falls, being struck by an object and motor vehicle crashes.174 Two ways to classify the severity of TBI are by the duration of loss of consciousness or post-traumatic amnesia175 and by the individual’s initial score on the 15-point Glasgow Coma Scale.176 • Mild TBI (also known as a concussion) is characterized by loss of consciousness or post-traumatic amnesia lasting 30 minutes or less, or an initial Glasgow score of 13 to 15; about 75% of TBIs are mild.177 • Moderate TBI is characterized by loss of consciousness or post-traumatic amnesia lasting more than 30 minutes but less than 24 hours, or an initial Glasgow score of 9 to 12. • Severe TBI is characterized by loss of consciousness or post-traumatic amnesia lasting 24 hours or more, or an initial Glasgow score of 8 or less. Overview 16 The risk of dementia increases with the number of TBIs sustained.173 Even those who experience mild TBI are at increased risk of dementia compared with those who have not had a TBI. A recent study found that mild TBI is associated with a two-fold increase in the risk of dementia diagnosis.178 Studies have also found that people with a history of TBI who develop Alzheimer’s do so at a younger age than those without a history of TBI.179-180 Whether TBI causes Alzheimer’s disease, other conditions that lead to dementia, or both, is still being investigated. Chronic traumatic encephalopathy (CTE) is a neuropathologic diagnosis (meaning it is characterized by brain changes that can only be identified at autopsy) associated with repeated blows to the head, such as those that may occur while playing contact sports. Among former amateur and professional football players, the risk of developing CTE, which is associated with dementia, increased 30% per year played.181 Currently, there is no test to determine if someone has CTE-related brain changes during life. A review article indicates that the greatest risk factor for developing CTE-related brain changes is repetitive brain trauma — repeated, forceful blows to the head that do not, individually, result in symptoms.182 Like Alzheimer’s disease, CTE is characterized by tangles of an abnormal form of the protein tau in the brain. Unlike Alzheimer’s, beta-amyloid plaques are uncommon in CTE.183-184 How the brain changes associated with CTE are linked to cognitive or behavioral changes is unclear. Other Modifiable Risk Factors Researchers are studying a variety of other potential modifiable risk factors. Among the many risk factors being studied are inadequate sleep or poor sleep quality,185 excessive alcohol use,186 depression187 and hearing impairment.188 In addition, a growing body of evidence indicates that critical illness and medical encounters such as hospitalization in older people increase their risk of long-term cognitive impairment.189-195 The emergence of novel coronavirus disease in 2019 (COVID-19) has resulted in tens of thousands of hospitalizations. This has the potential to increase the number of cases of cognitive impairment following critical illness. Furthermore, many of these individuals will receive or have received mechanical ventilation, which increases one’s risk of delirium,196 an acute state of short-term confusion that is a risk factor for dementia.197-199 There is also rapidly emerging evidence on how exposure to air pollution may be related to dementia risk. A number of different air pollutants have been studied in relation to cognition, cognitive decline, brain imaging and dementia itself. The most consistent and rigorous results concern fine particulate matter air pollution, which consists of tiny solid particles and liquid droplets generated by fuel combustion, fires and processes that produce dust.