Air Quality and Cognition

Overview

Air pollution and concerns about its effects continue to rise globally. However, policymakers and environmental regulators have neglected the effects of air pollution on cognitive functioning. A growing body of research points to the risk of exposure to high levels of pollution. It’s unclear how well all of the research will hold up under replication, but an abundance of studies point to clear detrimental effects of air pollution on cognition and decision-making.

Several individuals and organizations are concerned about the lesser-known effects of air pollution. For example, Matt Yglesias wrote an article in 2019 highlighting recent research on air quality’s effects on cognitive ability, productivity, decision-making, and dementia and Alzeihmer’s. Patrick Collison’s blog post on the issue appeared to inspire Ygelsias’s article.

Evidence suggests that air pollution has significant effects on both short-term and long-term cognition. While some studies used natural environmental variations to test the longer-term correlation between air pollution and cognition, other studies created isolated, laboratory experiments to test the short-term effects of air pollution on cognition. Both have yielded statistically significant results pointing to the negative effects of air pollution on cognition.

While air pollution clearly negatively affects cognition, its effects are nuanced and uneven. Cognition is an umbrella term encompassing many different domains of cognition. Generally, the six main domains of cognition are visuospatial/motor function, attention/concentration, learning/memory, executive functioning, social cognition/emotions, and language/verbal skills. Air pollution affects all domains of cognition, but the severity of the effect depends on brain matter, gender, age, and the affected domain of cognition.

In particular, multiple studies find stronger negative correlations between air quality and verbal test scores than math test scores. “Gray matter represents information processing centers, and white matter represents the networking of – or connections between – these processing centers. Mathematics abilities, which require more local processing, mainly depend on gray matter. While language skills, which require integrating and assimilating information from distributed gray-matter regions in the brain, mainly rely on white matter … A large body of literature has proven that air pollution can reduce the density of white matter more than that of grey matter in the brain, which may explain why air pollution appears to affect both verbal and math skills but in the meantime has a larger effect on verbal test than on math test scores” (Chen 2019).

The concentration of white and grey matter additionally creates gender differences in air quality’s cognitive effects. “Brain scanning studies reveal that men have a larger amount of gray matter activated during general intelligence tests than women do, but women have more white matter activated during general intelligence tests than men do. Given that gray matter is more required by math tests and white matter is more required by verbal tests, it is predicted that men’s cognitive performance, especially in the verbal domain, tends to be more affected by exposure to air pollution, while women’s cognition performance, especially in the math domain, is likely to be more affected” (Chen 2019). A study of older adults in China confirmed this. However, air pollution had a stronger effect on men’s verbal scores than women’s math scores.

The elderly are another demographic group heavily affected by air pollution. The same study of older adults in China also found that the effects of air pollution on test scores worsen with age. Furthermore, PM2.5 worsens short-term and long-term vulnerabilities to neurodegenerative diseases and neurodevelopmental disorders such as stroke, dementia, Alzhiemer’s disease, ASD, and Parkinson’s disease. (Fu 2019) found that “short- and long-term PM2.5 exposure was associated with increased risks of stroke (short-term odds ratio 1.01 [per 10 μg/m3 increase in PM2.5 concentrations], 95% CI 1.01-1.02; long-term 1.14, 95% CI 1.08-1.21) and mortality (short-term 1.02, 95% CI 1.01-1.04; long-term 1.15, 95% CI 1.07-1.24) of stroke. Long-term PM2.5 exposure was associated with increased risks of dementia (1.16, 95% CI 1.07-1.26), Alzheimer’s disease (3.26, 95% 0.84-12.74), ASD (1.68, 95% CI 1.20-2.34), and Parkinson’s disease (1.34, 95% CI 1.04-1.73).”

Research indicates that air quality has implications for people’s psychological wellbeing and happiness. In Edmonton, Canada, air pollution increased depression-related ED visits between 4.5 − 7.4% depending on the season, patients’ gender, and type of air pollutant (Szyszkowicz 2007). A study in China comparing local air quality and subjective well-being found that “a one SD decrease in API lifts happiness by 0.034 (equivalent to 0.036 SDs). The impact is rather sizable considering that a one SD increase in income status, one of the most important predictors of happiness, raises happiness by 0.039 (equivalent to 0.042 SDs).” Lastly, mice exposed to higher levels of air pollution exhibit increased levels of dopamine turnover and altered dopamine levels in their brains. These studies indicate that air pollution affects the domain of social cognition/emotions.

Decision-making suffers when air quality worsens. In chess tournaments, “an increase of 10 μg/m3 raises the probability of making an error by 1.5 percentage points, and increases the magnitude of the errors by 9.4%.” For Manhattan-based traders, “a one standard deviation increase in ambient PM2.5 concentrations reduces same-day returns by 11.9%.” Air quality even affects judgements in professional sports: “a 1 ppm increase in 3-hour CO causes an 11.5% increase in the propensity of umpires to make incorrect calls and a 10 mg/m3 increase in 12-hour PM2.5 causes a 2.6% increase.” Lastly, a study of individual decision-making in Beijing and PM2.5 levels “found increases in risk aversion to gains, risk tolerance over losses, ambiguity aversion over gains, and greater impatience in temporal discounting. In terms of other-regarding behavior, subjects became less prosocial, contributing less in a public goods game, reciprocating less in a sequential prisoners’ dilemma, and demanding more as responders in an ultimatum game.”

Air quality’s effects on cognition, decision-making, health, and productivity has important implications for developing countries. According to WHO, while “more than 80% of people living in urban areas that monitor air pollution are exposed to air quality levels that exceed the World Health Organization (WHO) limits… 98% of cities in low- and middle income countries with more than 100 000 inhabitants do not meet WHO air quality guidelines. However, in high-income countries, that percentage decreases to 56%.” Given that most of the existing evidence regarding this issue comes from developed countries with lower levels of air pollution, researchers and policymakers may be significantly underestimating the economic, social, and health consequences of air pollution.

Air pollution ultimately affects societies at multiple levels. Minor fluctuations in air quality can ripple out to have profound consequences. For high school students in Israel taking matriculation exams, PM2.5 concentrations affect their test scores and subsequent future earnings upwards of $30 per month. Air quality can influence important decisions involving one’s health, career, and investments. Lastly, air pollution is one of the global leading risk factors for death and leads to significant declines in economic productivity.

Background on Air Pollution

The most common air pollutants are particulate matter (PM), ozone (O), nitrogen dioxide (NO2) and sulfur dioxide (SO2)
“Particulate matter (PM) sizes range from coarse (PM10; <10μm) to fine (PM2.5; <2.5μm) to ultrafine (<0.1μm). While not a significant component of air pollution by mass, ultrafine particles (UFPs) achieve orders of magnitude higher particle count concentrations and surface area compared to larger particle sizes. For both chemical and health reasons, UFPs are generally considered among the most reactive elements of air pollution. Their high surface area permits greater adsorption of other toxic air pollutants per unit mass.” (Allen 2018)
For reference, the World Health Organization’s air quality guidelines recommend the following thresholds for exposure to various air pollutants to limit effects on human mortality and morbidity. However, their guidelines don’t account for effects on human cognition.
- PM_2.5: 10 μg/m3 annual mean; 25 μg/m3 24-hour mean
- PM₁₀: 20 μg/m3 annual mean; 50 μg/m3 24-hour mean
- 0₃: 100 μg/m3 8-hour mean
- N0₂: 40 μg/m3 annual mean; 200 μg/m3 1-hour mean
- S0₂: 20 μg/m3 24-hour mean; 500 μg/m3 10-minute mean
According to the EPA, average PM2.5 concentrations have decreased throughout the 21st century and have been below the EPA’s standard of 12 μg/m3 since 2012.
- In 2020, the cities with the highest annual PM2.5 concentrations were Fairbanks, AK (15.5 μg/m3), San Diego-Carslbad, CA (13.7 μg/m3), and Bakersfield, CA (13 μg/m3)

Cognitive Domains

Generally, the main domains of cognition can be lumped into the following 6 categories. However, the domains are not independent of each other.
1. Visuospatial and motor function
  1. Sensation and perception
  2. Motor skills and construction
2. Attention and concentration
  1. Processing speed
3. Learning and memory
4. Executive functioning
5. Social cognition/emotions
6. Language/verbal skills

Air Quality’s Effects on Cognition

The impact of exposure to air pollution on cognitive performance (Zhang and Chen 2018)

A 2018 paper estimated that “cutting annual mean concentration of particulate matter smaller than 10 μm (PM10) in China to the Environmental Protection Agency’s standard (50 μg/m3) would move people from the median to the 63rd percentile (verbal test scores) and the 58th percentile (math test scores), respectively.” (Zhang and Chen 2018)
The paper also found that “exposure to air pollution is associated with lower verbal test scores for both men and women regardless of the length of exposure. In general, the effect becomes larger as the duration of exposure to air pollution increases.” (See this figure for reference)
Air pollution affects verbal scores more than math scores. The study also found a statistically significant gender difference in their results: men experienced higher declines in their cognitive performance than women. (Zhang and Chen 2018)
The gender and verbal/math differences are likely driven by the concentration of white vs. grey matter in the brain. “Air pollution has a stronger effect on white matter (required more by verbal tests) than on gray matter (required more by math tests). Since men have a much smaller amount of white matter activated during intelligence tests, their cognitive performance, especially in the verbal domain, tends to be more affected by exposure to air pollution.”
The negative effects on verbal test performance are more pronounced (see figure) for older individuals, especially men. However, the pattern was less noticeable for math test performance.

Components of air pollution and cognitive function in middle-aged and older adults in Los Angeles (Gatto 2014)

In a study of 1496 adults in Los Angeles (mean age 60.5 years), “increasing exposure to PM2.5 was associated with lower verbal learning (β = −0.32 per 10 μg/m3 PM2.5, 95% CI = −0.63, 0.00; p = 0.05). Ambient exposure to NO2 >20 ppb tended to be associated with lower logical memory. Compared to the lowest level of exposure to ambient O3, exposure above 49 ppb was associated with lower executive function.”
“For PM2.5, the estimated decrement in verbal learning for each 10 μg/m3 exposure is approximately 30 percent greater than the expected decline associated with each additional year of age, and about 20 percent greater than the beta estimate associated with a high school education or less (compared to a graduate or professional degree).”

Fine Particulate Matter Air Pollution and Cognitive Function Among U.S. Older Adults (Ailshare and Clarke 2014)

Ailshare and Clarke found that air pollution has significant negative effects on cognition in adults older than 55 (N = 780).
“Those living in areas with greater exposure to PM_2.5 had an error rate 1.5 times greater than those exposed to lower PM_2.5 concentrations (IRR = 1.53 [95% CI, 1.02–2.30]). To put the difference in exposure to low–high pollution environments in context, the error rate of those with only 12 years of educational attainment was 1.5 times greater than those with more than 12 years of completed education (IRR = 1.52 [95% CI, 1.10–2.12])”

Neurobehavioral effects of ambient air pollution on cognitive performance in US adults (Chen and Shwartz 2009)

Chen and Shwartz measured air pollution’s neurobehavioral effects on 1764 adults. They used a symbol-digit substitution test (SDST) to measure participant’s coding abilities, a simple reaction time test (SRTT) to test reaction speed, and a serial-digit learning test (SDLT) to measure their attention and short-term memory.
“Each 10-ppb increase in annual ozone was associated with increased SDST and SDLT scores by 0.16 (95%CI: 0.01, 0.23) and 0.56 (95%CI: 0.07, 1.05), equivalent to 3.5 and 5.3 years of aging-related decline in cognitive performance.”
While estimated annual ozone levels consistently predicted reduced SDST scores and SDLT scores, they did not predict SRTT scores (i.e. ozone levels predicted reduced coding, attention, and short-term memory scores, but did not predict reaction speeds). The authors hypothesize that air pollution primarily affects memory and attention functions within the central nervous system.

Effects of short-term exposure to particulate matter air pollution on cognitive performance (Shehab and Pope 2019)

Shebab and Pope tested the cognitive effects of short-term exposure to air pollution through two distinct tests. (The subjects were primarily students under 24.)
“Firstly, subjects completed a series of cognitive tests after being exposed to low ambient indoor PM concentrations and elevated PM concentrations generated via candle burning, which is a well-known source of PM. Secondly, a different cohort underwent cognitive tests after being exposed to low ambient indoor PM concentrations and elevated ambient outdoor PM concentrations via commuting on or next to roads.”
The study used the Mini-Mental State Examination (MMSE), Stroop Word-Color test, and Ruff 2 and 7 Selective Attention Test to respectively test cognitive functioning, executive functioning, and visual attention (i.e. sustained attention, and selective attention).
Exposure to burning candles had a statistically significant, adverse effect on the mean T-score in the MMSE test. Subjects also experienced greater declines in cognitive performance when exposed to higher levels of PM_2.5. There was no statistically significant effect of candle burning on Stroop Word-Color test and Ruff 2 and 7 test scores.
Exposure to pollutants from commuting again had a statistically significant effect on the mean T-score in the MMSE test. Again, there was no statistically significant effect of commuting on Stroop Word-Color test. Regarding the Ruff 2 and 7 test, “no cognitive decline was observed in any of the assessed domains except for selective attention—automatic detection speed, where the p-value was 0.006, providing very strong evidence for short term cognitive decline.”

Smog, Cognition and Real-World Decision-Making (Chen 2019)

“First, pollution exposure may increase risk for strokes and then vascular dementia16; second, people breathing polluted air are more likely to be oxygen deficient, which in turn impairs their cognitive abilities17; third, exposure to pollution leads to the growth of white-matter lesions, potentially inhibiting cognition18; fourth, air pollution may degrade cognitive ability via asthma and respiratory problems, persistently constraining the production of human capital, such as schooling19 and labor force participation20; finally, air pollution may also damage the immune system, hinder neurological development, and impair neuron behavior, which all contribute to long-term memory formation.16”
“Recent economic studies also show air pollution may disrupt cognitive functioning through psychological pathways. For example, high concentration of pollutants is significantly associated with headache,21cause psychiatric distress,22 and increase the risk of feeling unhappy and depressed in the United States,23 Canada,24 and China.25 PM2.5 is currently the only pollutant among key atmospheric pollutants in the Air Quality Index (AQI) evidenced to cause psychological disorders.26”

Ambient air pollution and neurotoxicity on brain structure: Evidence from women’s health initiative memory study (Chen 2015)

“Older women with greater PM2.5 exposures had significantly smaller WM, but not GM, volumes, independent of geographical region, demographics, socioeconomic status, lifestyles, and clinical characteristics, including cardiovascular risk factors. For each interquartile increment (3.49μg/m(3) ) of cumulative PM2.5 exposure, the average WM volume (WMV; 95% confidence interval) was 6.23cm(3) (3.72-8.74) smaller in the total brain and 4.47cm(3) (2.27-6.67) lower in the association areas, equivalent to 1 to 2 years of brain aging. The adverse PM2.5 effects on smaller WMVs were present in frontal and temporal lobes and corpus callosum (all p values <0.01). Hippocampal volumes did not differ by PM2.5 exposure.”

The Long-Run Economic Consequences of High-Stakes Examinations: Evidence from Transitory Variation in Pollution (Ebenstein 2016)

After examining the relationship between PM levels and test scores from 400,000 administrations of the Bagrut (Israel’s standardized test), the authors found “a 1 standard deviation increase in the PM2.5(Air Quality Index (AQI)) is associated with a decline in student performance of 0.93 points, or 3.9 percent of a standard deviation (σBagrut = 23.74)
The estimated magnitude is larger for boys, weaker students, and students from lower socioeconomic backgrounds.
“We estimate that an additional ten units of PM2.5(AQI) exposure across the student’s exams is associated with a 1.64 unit decline in a student’s Bagrut composite score, a 0.15 decline in years of education at a university, and a 109 Israeli shekels ($30) decline in monthly salary.”
“We find that each additional instrumented point increases postsecondary academic enrollment by 1.9 percentage points, postsecondary education by 0.092 years, and 66 shekels (or 1.3 percent) in wages. Interestingly, we also find there is virtually no effect of pollution on noncompetitive forms of higher education (e.g., technical schools). This suggests that the mechanism for the Bagrut’s impact on student outcomes is through the posited channel of affecting a student’s prospects for competitive postsecondary education.”
“We find that the return to a Bagrut point is larger for boys than for girls (78 shekels versus 59 shekels), for stronger students than weaker students (124 shekels versus 80 shekels), and for higher socioeconomic status students than lower socioeconomic status students (105 shekels versus 56 shekels).”

Exposure to Particulate Air Pollution and Cognitive Decline in Older Women (Weuve 2013)

A study of 19,409 US women aged 70 to 81 years found that “higher levels of long-term exposure to both PM2.5-10 and PM2.5 were associated with significantly faster cognitive decline. Two-year decline on a global score was 0.020 (95% CI, −0.032 to −0.008) standard units worse per 10 μg/m3 increment in PM2.5-10 exposure and 0.018 (95% CI, −0.035 to −0.002) units worse per 10 μg/m3 increment in PM2.5 exposure. These differences in cognitive trajectory were similar to those between women in our cohort who were approximately 2 years apart in age, indicating that the effect of a 10-μg/m3 increment in long-term PM exposure is cognitively equivalent to aging by approximately 2 years.”

Decision Making

The Impact of Indoor Climate on Human Cognition: Evidence from Chess Tournaments (Kunn 2019)

“The results indicate air pollution (PM2.5) is the deterring factor hindering cognitive performance. We find that an increase of 10 μg/m3 raises the probability of making an error by 1.5 percentage points, and increases the magnitude of the errors by 9.4%. The impact of pollution is exacerbated by time pressure. When players approach the time control of games, an increase of 10 μg/m3, corresponding to about one standard deviation, increases the probability of making a meaningful error by 3.2 percentage points, and errors being 17.3% larger.”

The Effect of Air Pollution on Investor Behavior: Evidence from the S&P 500 (Heyes 2016)

“When Manhattan-based traders are exposed to higher levels of PM2.5, the return on the S&P 500 on that day is lowered. This is consistent with an induced fall in risk appetite among a subset of traders, plausible given existing research on the role short-term exposure to air pollution can have on brain health, cognition and risk attitude. The effects are economically significant—a one standard deviation increase in ambient PM2.5 concentrations reduces same-day returns by 11.9% in our preferred specification—and prove robust to a variety of specifications and a battery of robustness and falsification checks.”

Air Quality and Error Quantity: Pollution and Performance in a High-Skilled, Quality-Focused Occupation (Archsmith 2018)

“We provide the first evidence that short-term exposure to air pollution affects the work performance of a group of highly skilled, quality-focused employees. We repeatedly observe the decision making of individual professional baseball umpires, quasi-randomly assigned to varying air quality across time and space. Unique characteristics of this setting combined with high-frequency data disentangle effects of multiple pollutants and identify previously underexplored acute effects. We find that a 1 ppm increase in 3-hour CO causes an 11.5% increase in the propensity of umpires to make incorrect calls and a 10 mg/m3 increase in 12-hour PM2.5 causes a 2.6% increase. We control carefully for a variety of potential confounders, and re- sults are supported by robustness and falsification checks. Our estimates imply that a 3% reduction in productive output is associated with a change in CO concentrations equivalent to moving from the 25th to the 95th percentile of the CO distribution in many of the largest US cities.”

Does Haze Cloud Decision Making? (Hong 2019)

Abstract: “The adverse impact of haze on health and its association with a range of economic outcomes have received increasing attention in the literature. A natural laboratory experiment involving more than 600 subjects enables a first attempt at investigating the causal effect of haze, proxied by particulate matter of up to 2.5 microns in diameter (PM2.5) on decision making. This study was conducted in Beijing in October 2012 over five days with highly varying levels of PM2.5, which only became commonly known in China in 2013. We observed several effects associated with an increase in the level PM2.5. In terms of individual decision making, we found increases in risk aversion to gains, risk tolerance over losses, ambiguity aversion over gains, and greater impatience in temporal discounting. In terms of other-regarding behavior, subjects became less prosocial, contributing less in a public goods game, reciprocating less in a sequential prisoners’ dilemma, and demanding more as responders in an ultimatum game. Our results underpin several reported findings in the literature linking short-term variations in air quality to real-world economic variables, including stock market performance, worker productivity, movie attendance and revenue, criminal activities, and subjective wellbeing.”

Psychological Costs

The effects of exposure to air pollution on subjective wellbeing in China (Zhang and Chen 2020)

The authors match a nationally representative survey in China with local air quality and weather conditions according to the exact date and county of each interview. By making use of variations in exposures to air pollution across similar respondents living in the same county, the authors find that particulate matter with a diameter smaller than 2.5 micrometers (PM2.5) reduces hedonic happiness and increases the rate of depressive symptoms, but does not affect life satisfaction.

Happiness in the Air: How Does a Dirty Sky Affect Mental Health and Subjective Well-being? (Zhang and Chen 2017)

“a one SD decrease in API lifts happiness by 0.034 (equivalent to 0.036 SDs). The impact is rather sizable considering that a one SD increase in income status, one of the most important predictors of happiness, raises happiness by 0.039 (equivalent to 0.042 SDs). In the CCTV Postcard Survey, self-reported happiness declined by 0.326 SDs (measured by the SD in 2014) from 2007 to 2014. The 37.132 units or 2.036-standard-deviation (measured by the SD in 2014) increase in population-weighted annual mean API during the same period accounts for 22.5 percent of the actual decrease in happiness.”
“The coefficient on API indicates that a one-unit increase in API leads to a decline in happiness by 0.044%, while the coefficient on log per capita income shows that a 1% increase in annual household per capita income raises happiness by 0.022%. According to a back-of-the-envelope calculation, people are on average willing to pay 2.0% of their annual income for a one-unit reduction in API on the day of the interview.”

Air pollution and emergency department visits for depression in Edmonton, Canada

(Szyszkowicz 2007)

“After adjusting for temperature and relative humidity, the following increments in daily depression-related ED visits could be noted: 6.9% (95% CI: 1.3, 12.9) for carbon monoxide (CO) for all patients in warm season; 7.4% (95% CI: 0.5, 14.8) for nitrogen dioxide (NO2) for female patients in warm season; 4.5% (95% CI: 0.1, 9.1) for sulphur dioxide (SO2) for female patients in warm season; 6.9% (95% CI: 0.6, 13.6) for ground level ozone (O3, 1-day lagged) for female patients in warm season; 7.2% (95% CI: 2.7, 12.0) for particulate matter (PM10) for females in cold season; and 7.2% (95% CI: 2.0, 12.8) for particulate matter (PM2.5) for females in cold season.”

Impact on Economic Productivity

The Impact of Pollution on Productivity (Zivin and Neidell 2011)

This study measuring the productivity of 1600 agricultural workers found that a “1 ppb increase in ozone leads to a statistically significant decrease in productivity of .012 of a standard deviation.” Furthermore, their estimate “implies that a 10 ppb decrease in ozone increases worker productivity by 4.2 percent.”
There are also multiple other papers about air quality’s effects on economic productivity.

Dementia & Alzheimer’s

The association between PM 2.5 exposure and neurological disorders: A systematic review and meta-analysis (Fu 2019)

“We found significant association between PM2.5 exposure and stroke, dementia, Alzheimer’s disease, ASD, Parkinson’s disease. The risks of ischemic and hemorrhagic stroke were higher than that of stroke in general, and that hemorrhagic stroke had by far the highest mortality. The risk of stroke for heavily polluted countries was significantly higher than that of lightly polluted countries. Short- and long-term PM2.5 exposure was associated with increased risks of stroke (short-term odds ratio 1.01 [per 10 μg/m3 increase in PM2.5 concentrations], 95% CI 1.01-1.02; long-term 1.14, 95% CI 1.08-1.21) and mortality (short-term 1.02, 95% CI 1.01-1.04; long-term 1.15, 95% CI 1.07-1.24) of stroke. Long-term PM2.5 exposure was associated with increased risks of dementia (1.16, 95% CI 1.07-1.26), Alzheimer’s disease (3.26, 95% 0.84-12.74), ASD (1.68, 95% CI 1.20-2.34), and Parkinson’s disease (1.34, 95% CI 1.04-1.73).”

Hazed and Confused: The Effect of Air Pollution on Dementia (Bishop 2019)

“We find that long-term exposure to fine-particulate air pollution (PM2.5) degrades health and human capital among older adults by increasing their risk of developing Alzheimer’s disease and related dementias. We track U.S. Medicare beneficiaries’ cumulative residential exposures to PM2.5 and their health from 2004 through 2013, leveraging within- and between-county quasi-random variation in PM2.5 resulting from the expansion of Clean Air Act regulations. We find that a 1 ig/m3 increase in decadal PM2.5 increases the probability of a dementia diagnosis by 1.68 percentage points. The effects are as large or larger when we adjust for mortality-based sample selection and additional Tiebout-sorting dynamics. We do not find relationships between decadal PM2.5 and placebo outcomes. Our estimates suggest that the federal regulation led to nearly 182,000 fewer people with dementia in 2013, yielding $214 billion in benefits. Further, PM2.5’s effect on dementia persists below the current regulatory thresholds”