--- topic: "Outdoor Air Pollution" title: "Outdoor Air Pollution and Your Body" order: 1 description: "Fine-particle pollution raises heart, lung, and lung-cancer risk and may add to dementia risk, even below legal limits. How big it is and what helps." question: "How does outdoor air pollution affect your heart, lungs, and brain?" answer: "Fine-particle pollution probably raises the risk of heart disease, lung disease, lung cancer and, less clearly, dementia, even below legal limits." actions: - "Check the Air Quality Index before outdoor exercise (AirNow site and app in the US): above 100, sensitive groups should ease off, and above 150 everyone may notice effects [1]." - "Keep exercising, as benefits outweigh pollution risks in most cities, but pick parks and quiet streets over busy roads: a two-hour busy-London-street walk blunted gains seen in a park [2][3]." - "On high-AQI days, consider moving workouts indoors [1]; the American Heart Association says evidence for many personal measures is limited, so no single step erases the risk [4]." cautions: - "Older adults, heart or lung disease: in COPD, a busy-street walk caused more cough, sputum and wheeze than a park walk; ask a clinician for a plan [2]." - "Children's lungs respond to air quality: lung-function growth improved as southern California air got cleaner, so a child with asthma or frequent wheeze deserves extra attention on bad-air days [5]." confidence: 4 confidenceNote: "Large US and European cohorts agree on mortality and heart disease, and cleaner-air natural experiments fit a causal link, but no long-term-exposure randomized trial exists and dementia evidence is weaker." evidenceSummary: rctCount: 1 observationalCount: 5 metaAnalysisCount: 2 totalParticipants: "60 million+ in the largest cohort; 325,000+ in the European pooled analysis" replicated: true newestStudyYear: 2026 references: - id: "1" url: "https://www.airnow.gov/aqi/aqi-basics/" title: "AQI Basics" authors: "U.S. Environmental Protection Agency, AirNow" year: 2026 journal: "AirNow.gov" - id: "2" pmid: "29221643" title: "Respiratory and cardiovascular responses to walking down a traffic-polluted road compared with walking in a traffic-free area in participants aged 60 years and older with chronic lung or heart disease and age-matched healthy controls: a randomised, crossover study" authors: "Sinharay R, Gong J, Barratt B, et al." year: 2018 journal: "Lancet" - id: "3" pmid: "27156248" title: "Can air pollution negate the health benefits of cycling and walking?" authors: "Tainio M, de Nazelle AJ, Götschi T, et al." year: 2016 journal: "Prev Med" - id: "4" pmid: "33150789" title: "Personal-Level Protective Actions Against Particulate Matter Air Pollution Exposure: A Scientific Statement From the American Heart Association" authors: "Rajagopalan S, Brauer M, Bhatnagar A, et al." year: 2020 journal: "Circulation" - id: "5" pmid: "25738666" title: "Association of improved air quality with lung development in children" authors: "Gauderman WJ, Urman R, Avol E, et al." year: 2015 journal: "N Engl J Med" - id: "6" pmid: "17267905" title: "Long-term exposure to air pollution and incidence of cardiovascular events in women" authors: "Miller KA, Siscovick DS, Sheppard L, et al." year: 2007 journal: "N Engl J Med" - id: "7" pmid: "24911630" title: "Outdoor particulate matter exposure and lung cancer: a systematic review and meta-analysis" authors: "Hamra GB, Guha N, Cohen A, et al." year: 2014 journal: "Environ Health Perspect" - id: "8" pmid: "28063597" title: "Living near major roads and the incidence of dementia, Parkinson's disease, and multiple sclerosis: a population-based cohort study" authors: "Chen H, Kwong JC, Copes R, et al." year: 2017 journal: "Lancet" - id: "9" pmid: "34558969" title: "Exposure to Air Pollution in Relation to Risk of Dementia and Related Outcomes: An Updated Systematic Review of the Epidemiological Literature" authors: "Weuve J, Bennett EE, Ranker L, et al." year: 2021 journal: "Environ Health Perspect" - id: "10" pmid: "28657878" title: "Air Pollution and Mortality in the Medicare Population" authors: "Di Q, Wang Y, Zanobetti A, et al." year: 2017 journal: "N Engl J Med" - id: "11" pmid: "34470785" title: "Long term exposure to low level air pollution and mortality in eight European cohorts within the ELAPSE project: pooled analysis" authors: "Strak M, Weinmayr G, Rodopoulou S, et al." year: 2021 journal: "BMJ" - id: "12" pmid: "38762324" title: "Global burden and strength of evidence for 88 risk factors in 204 countries and 811 subnational locations, 1990-2021: a systematic analysis for the Global Burden of Disease Study 2021" authors: "GBD 2021 Risk Factors Collaborators" year: 2024 journal: "Lancet" --- Fine particles in outdoor air (PM2.5) raise the risk of heart disease, lung disease, and lung cancer, and may add to dementia risk [5][6][7][8][9]. The extra risk for one person is modest (about 7% higher mortality per 10 µg/m³ in older Americans), but it applies to millions of people and continues below legal limits [10][11]. You can lower your own dose by checking the Air Quality Index, avoiding busy roads when you exercise, and staying active rather than staying in [1][2][3]. ## How it works **What is measured.** PM2.5 means particles smaller than 2.5 micrometres across. The US Air Quality Index (AQI) turns pollutant readings, including ozone and particles, into a six-level scale where 100 matches the short-term health standard [1]. **Heart.** In 65,893 postmenopausal women in 36 US metro areas, each 10 µg/m³ rise in PM2.5 went with 24% more cardiovascular events and 76% more cardiovascular deaths over a median six years [6]. **Lungs.** As southern California air improved over 13 years, the share of 15-year-olds with low lung function fell from 7.9% to 3.6% [5]. In older adults with COPD, a two-hour walk on a London shopping street caused more cough, sputum, and wheeze than a walk in a park [2]. **Cancer.** The International Agency for Research on Cancer classes outdoor particulate matter as a Group 1 carcinogen, and a meta-analysis of 18 studies found 9% higher lung cancer risk per 10 µg/m³ of PM2.5 [7]. **Brain.** Living within 50 metres of a major road went with 7% higher dementia incidence [8]. A review of 66 papers found the weight of evidence suggests PM2.5 contributes to cognitive decline, but strong conclusions remain elusive [9]. **How big is the risk?** Among 60 million Medicare enrollees, each 10 µg/m³ rise in PM2.5 went with 7.3% higher all-cause mortality, and the link continued below the US standard [10]. A pooled European analysis agreed at low levels [11]. Particle pollution was the leading single contributor to global disease burden in 2021, at 8.0% of disability-adjusted life years [12]. Those are relative risks over years of exposure and population-wide shares, not a verdict on any one day or person. **What helps.** Walking and cycling usually win: at the global average urban PM2.5 of 22 µg/m³, activity benefits far outweigh pollution risks, and harms pass benefits only around 100 µg/m³ and after about 1.5 hours of cycling a day [3]. Where you walk matters: a park walk improved lung function and arterial stiffness for up to 26 hours while a busy-street walk blunted it [2]. At home, filtration helps with particles; see [Do Air Purifiers Actually Improve Health?](/air-quality/do-air-purifiers-improve-health), and for smoke days, [Protecting Yourself from Wildfire Smoke](/wildfire-smoke/protecting-yourself-from-wildfire-smoke). The American Heart Association calls the causal role of PM2.5 in cardiovascular disease unequivocal, yet the evidence for many personal measures limited [4]. ## The evidence in detail **All-cause mortality.** Di and colleagues followed 60,925,443 Medicare beneficiaries from 2000 to 2012 (460 million person-years) using ZIP-code exposure models [10]. A 10 µg/m³ increase in PM2.5 raised all-cause mortality by 7.3% (95% CI 7.1 to 7.5), and a 10 ppb increase in ozone by 1.1% [10]. Restricting to years below 12 µg/m³ and 50 ppb gave 13.6% (13.1 to 14.1) for PM2.5; effects were larger in racial minorities and people with low income [10]. The ELAPSE project pooled eight European cohorts (325,367 adults, 19.5 years, 47,131 deaths): 5 µg/m³ more PM2.5 went with 13% (10.6 to 15.5) more natural-cause deaths, and 29.6% (14 to 47.4) among those below 12 µg/m³ [11]. **Cardiovascular disease.** The Women's Health Initiative cohort of 65,893 women without prior heart disease found a hazard ratio of 1.24 (1.09 to 1.41) for a first cardiovascular event and 1.76 (1.25 to 2.47) for cardiovascular death per 10 µg/m³, with within-city differences mattering [6]. **Lungs.** The Children's Health Study measured lung function in 2,120 children from three cohorts (1994-98, 1997-2001, 2007-11) [5]. Falling nitrogen dioxide and PM2.5 went with better FEV1 and FVC growth between ages 11 and 15, in children with and without asthma; low FEV1 at 15 fell from 7.9% to 6.3% to 3.6% [5]. In the Lancet crossover trial, 119 older adults (40 healthy, 40 with COPD, 39 with ischaemic heart disease) walked two hours on Oxford Street and in Hyde Park [2]. COPD participants reported more sputum (odds ratio 3.15, 1.39 to 7.13) and wheeze (4.00, 1.52 to 10.50) after Oxford Street [2]. Everyone gained lung function and lower arterial stiffness after the park, and those gains were attenuated on the busy street [2]. **Lung cancer.** Hamra and colleagues pooled 18 studies: relative risk 1.09 (1.04 to 1.14) per 10 µg/m³ PM2.5, 1.18 (1.00 to 1.39) in never-smokers, and 1.40 (1.07 to 1.83) for adenocarcinoma [7]. **Dementia.** In Ontario, about 2.2 million adults aged 55 to 85 were followed from 2001 to 2012, with 243,611 incident dementia cases [8]. The hazard ratio was 1.07 (1.06 to 1.08) within 50 m of a major road, 1.04 within 50 to 100 m, and 1.00 beyond 200 m, stronger in big cities (1.12) [8]. A systematic review of 66 papers, 35 meeting strict quality criteria, supported an adverse association between PM2.5 and cognitive decline, but PM10 and nitrogen oxide findings were inconclusive, and few high-quality studies examined dementia itself [9]. **Exercise trade-offs.** A modelling study estimated that at 22 µg/m³ the benefits of cycling and walking far outweigh the risks even at extreme activity levels [3]. At 100 µg/m³, harms exceed benefits after about 1.5 hours of cycling or 10 hours of walking a day, and results were sensitive to dose-response assumptions [3]. **Null and conflicting results.** Current smokers showed no clear lung-cancer link (1.06, 0.97 to 1.15) [7]. The road-proximity link is small and absent for Parkinson's disease and multiple sclerosis [8]. Dementia reviews flag selection bias and reliance on administrative records [9]. All cohort exposures are modelled estimates at a home address, which misclassifies individual exposure, and no randomized trial of long-term exposure exists, so causation rests on consistency across cohorts, natural experiments, and mechanism [5][10]. We found no trial of time of day for outdoor exercise. **Bottom line (confidence 4 of 5).** The population-level harm of fine-particle pollution is well established for mortality, heart disease, and lung cancer, and probable for cognitive decline. The relative increase per person is modest, and staying active still wins in most places.