Air Pollution & Heart Failure: Hospital Readmissions Show Clear Link

 In Cardiopulmonary Medicine


Consider how cause and effect might apply to mouse traps. It is easy to assume that mouse traps are associated with greater murine mortality, and based on this, you might advise mice to avoid them. The association is clearly time-dependent: There will be little impact on mortality if the mouse avoids a mouse trap after the trap has already sprung. However, if the mouse is caught in the trap, your advice is too late. I bring this up because some of the associations we’ve learned about from epidemiological research may be similar; once an illness has occurred, avoidance of the causal triggers associated with the illness may not have benefit. It is not easy to know. 

PM2.5 Linked to Heart Failure Hospitalizations 

These thoughts were triggered by a study by Ward-Caviness et al, published this year in the Journal of the American Heart Association.1 The researchers used some fancy statistical tests to compare chronic exposure to ultra-fine air pollutant particulates (PM2.5) in heart failure patients with their risk of being readmitted to the hospital after an initial cardiac event. [PM2.5 refers to particles less than 2.5 µm in diameter.] To accomplish this analysis, the researchers accessed population data through “EPA CARES” (the Environmental Protection Agency’s Clinical and Archived Records Research for Environmental Studies). EPA CARES is a treasure trove of electronic health records merged with environmental exposure data gathered to facilitate environmental health studies. The researchers analyzed data from a total of 20 920 heart failure patients and used sophisticated regression models to associate annual average fine particulate matter at the date of heart failure (HF) diagnosis with the number of hospital visits and 30‐day readmissions.1 First, they individually geocoded each patient’s address and estimated their daily exposure to PM2.5. Electronic health records were then used to determine their initial diagnosis of HF and how many times they returned to the hospital. A total of 442 244 hospital visits were counted for these patients over an average follow‐up period of 2.79 years.1   

To no one’s great surprise, the greater the exposure to air pollution, the worse the patients fared. A 1 µg/m3 increase in fine particulate matter was associated with a 9.31% increase in total hospital visits (95% CI, 7.85%–10.8%), a 4.35% increase in inpatient admissions (95% CI, 1.12%–7.68%), and a 14.2% increase in 30‐day readmissions (95% CI, 8.41%–20.2%). These associations remained robust in the face of all sorts of statistical modeling approaches.1  

This should be considered a big deal. Heart failure rates are increasing. By 2030, an estimated 8 million individuals in the United States will have HF, a 46% increase from 2012.2 At that time, the total costs of HF were estimated at $30.7 billion, approximately 68% of which were direct healthcare costs such as hospital visits and inpatient stays. By 2030, it is estimated that the total cost of HF will increase to $69.8 billion – a 127% increase.2 Tracking readmission rates has become important, in part because the Affordable Care Act imposes financial fines on hospitals for poor performance. Hospitals with elevated 30-day readmission rates may have 3% of the Medicare and Medicaid fee for service payments for heart failure withheld. That may not sound like much, but it added up to hundreds of millions of dollars in 2020. While we may not be that empathetic to the financial concerns of large corporate entities, readmission rates are a valuable calculus to use in assessing efficacy of interventions and therapies. 

Racial Health Disparities & PM2.5 Exposure 

As a side note, in the Ward-Caviness study, Black patients showed the highest association between total hospital visits and long‐term air pollution exposure; their risk was 40% higher than that of White patients. This racial disparity was even larger for 30‐day readmissions. Minority populations are more likely to live in urban areas, and, as such, are often exposed to higher-than-average levels of air pollution. It is thus possible that some of the well-documented racial health disparities seen in HF patients are driven by the degree of exposure to air pollution.3  

A similar explanation may underlie the disparities in breast cancer among Black women. High PM2.5 exposure is associated with greater risk for more aggressive forms of breast cancer.4   

PM2.5 & Hospitalizations for Anything 

There is no longer doubt that air pollution raises the risk of being admitted to the hospital, in general. A 2019 study by Yazdi et al reported that long‐term exposure to PM2.5 was associated with an increased inpatient admission among Medicare recipients, even in areas where PM2.5 concentrations were below the National Ambient Air Quality Standard of 12 µg/m.5 The aforementioned Ward-Caviness study only looked at patients with existing heart failure, and suggested a similar association. 

The biological mechanisms behind these associations are well understood, and include systemic inflammation, increased activation of the autonomic nervous system, and oxidative stress induced by penetration of PM2.5 particles into the respiratory tract.6-8  

What to Tell Patients 

The data from Ward-Caviness and similar studies deserve consideration, and we may be inspired to encourage heart failure patients to lower their PM2.5 exposure as much as possible. Yet, this is where my ruminations about mouse traps comes in. As far as I am aware, we don’t have interventional trials to confirm the idea that HF patients will do better with clean air post-diagnosis. While it may seem obvious that if bad air causes a disease, clean air may improve it, that is just our assumption, being the naturopathic practitioners we are. But what if heart disease is a 1-way street? Factors that contribute to HF may build up to a breaking point, and once the disease process is initiated, there is no way to reverse it. One can light a fire with a match, but once it’s burning, blowing out the match won’t put out the fire. Could heart failure be similar? 

This is a common problem in epidemiologic research. There is no easy test to determine whether a perceived association is causative or not. There are the Bradford Hill criteria, of course, used to evaluate associations, but they are not hard and fast; to paraphrase Captain Barbossa in the movie Pirates, these criteria are  “… more what you’d call ‘guidelines’ than actual rules.” 

The only way to tell if an association is causational is a clinical trial that explores whether an intervention will prevent a disease. But once the disease is present, we still don’t know whether the same strategy of intervention will reverse the disease process. Will it be like a mouse trap, in that once the trap is sprung, there is no going backwards? Patients may not be helped by this sort of nitpicking. Clean air is healthier than polluted air, and the tiniest of particles are now understood to trigger the worst problems. That may be all we need to share. In my mind, any patient with heart problems should be told that fine-particulate air pollution may worsen their prognosis, though admittedly without evidence from interventional clinical trials. And we may never know this, as conducting such a trial in humans presents obvious ethical challenges. 

Make Your Stand Inside  

Although the majority of the PM2.5 burden originates “outdoors,” most exposure occurs indoors.9 The US mortality burden associated with PM2.5 exposure was estimated in 2012 to be between 230 000 and 300 000 deaths.9 Indoor exposure may account for as much as 60% of this total.9   

Various studies have reported on the effectiveness of indoor air filtration. In a Beijing study, average indoor PM2.5 dropped from 60 to 24 μg/m3.10 A Shanghai trial reported a drop from 96.2 to 41.3 μg/m3 following 2 weeks of active air filtration – a drop of 57%.11 Obviously, such cities have rather dirty air. In a Danish study, conducted in Copenhagen where the air is relatively clean, 2 weeks of filtration still reduced PM2.5 by half, from 8 to 4 μg/m3.12 A similar 40% decrease in PM2.5 levels was seen in a Canadian study.13 A trial conducted in Detroit, Michigan, reported that portable air filters reduced PM2.5 exposure by more than 50%.14 So, based on these findings, we can assume that using a HEPA filter will decrease PM2.5 levels by about half. A little effort can make a significant difference in air quality 

Rough Calculations of Benefit 

Practitioners of all stripes should be actively encouraging patients to be proactive in reducing their own exposure. And these new data regarding congestive heart failure and PM2.5 should be used to specifically encourage patients with cardiovascular disease to do so.  

According to the World Bank’s database, average exposure to PM2.5 has fallen in the United States, from 9.741 µg/m3 in 2011, down to 7.41 µg/m3 in 2017.15 The EPA provides up-to-date information on pollution levels by zip code at .16 If this rough approximation holds true that home-filters drop exposure level by half, say a further 3.5 µg/m3, then this simple intervention might reduce hospital visits for those suffering from heart failure by approximately one-third: (3.5 µg/m3 decrease in PM2.5) x (9.3% change in hospital visits/µg/m3) = 32.55% decrease in hospital visits.   

For all of us, including our patients, it’s not about reducing healthcare costs, but about reducing suffering and improving quality of life. These numbers suggest that effort spent reducing exposure to air pollutants might do both to a significant degree. 

[Insert sidebar somewhere in article]: 

Car Cabin Filters 

Modern cars have a replaceable cabin filter for cleaning the air circulating inside the car. These are usually tucked behind the glove compartment and are easy to replace. In fact, they are supposed to be replaced at yearly intervals, though most people are unaware that they even exist (and some of us forget to replace them). One can purchase HEPA versions of these filters that remove even the fine particulates from the air inside the car. One 2018 study reported that PM2.5 levels measured inside cars on the highway were 133.9 µg/m3.17 

In the same year, Vande Hey et al examined the effectiveness of an experimental air scrubber designed to remove ultra-fine particulates inside cars. Their study found that the filter could drop PM2.5 concentrations from 100 µg/m3 to below 25 µg/m3 in less than 2 minutes, and to 5 µg/m3 in less than 5 minutes.18 Currently available HEPA cabin air filters may not be this effective, but they are better than the old clogged filters that so many of us and our patients are driving with in our cars. The HEPA filters cost about $30 and require little skill to install. Given the potential benefit, replacing this filter is a reasonable task to put on any patient’s to-do list, but especially those with heart problems.  


  1. Ward-Caviness CK, Danesh Yazdi M, Moyer J, et al. Long-Term Exposure to Particulate Air Pollution Is Associated With 30-Day Readmissions and Hospital Visits Among Patients With Heart Failure. J Am Heart Assoc. 2021;10(10):e019430.  
  1. Virani SS, Alonso A, Benjamin EJ, et al. Heart disease and stroke statistics‐2020 update: a report from the American Heart Association. Circulation. 2020;141(9):e139-e596. 
  1. Yitshak-Sade M, Lane KJ, Fabian MP, et al. Race or racial segregation? Modification of the PM2.5 and cardiovascular mortality association. PLoS One. 2020;15(7):e0236479.  
  1. Prada D, Baccarelli AA, Terry MB, et al. Long-term PM2.5 exposure before diagnosis is associated with worse outcome in breast cancer. Breast Cancer Res Treat. 2021;188(2):525-533.  
  1. Danesh Yazdi M, Wang Y, Di Q, et al. Long‐term exposure to PM2.5 and ozone and hospital admissions of Medicare participants in the Southeast USA. Environ Int. 2019;130:104879.  
  1. Fiordelisi A, Piscitelli P, Trimarco B, et al. The mechanisms of air pollution and particulate matter in cardiovascular diseases. Heart Fail Rev. 2017;22(3):337-347. 
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  1. Brook RD, Rajagopalan S, Pope CA 3rd, et al. American Heart Association Council on Epidemiology and Prevention, Council on the Kidney in Cardiovascular Disease, and Council on Nutrition, Physical Activity and Metabolism. Particulate matter air pollution and cardiovascular disease: An update to the scientific statement from the American Heart Association. Circulation. 2010;121(21):2331-2378.  
  1. Azimi P, Stephens B. A framework for estimating the US mortality burden of fine particulate matter exposure attributable to indoor and outdoor microenvironments. J Expo Sci Environ Epidemiol. 2020;30(2):271-284.  
  1. Shao D, Du Y, Liu S, et al. Cardiorespiratory responses of air filtration: A randomized crossover intervention trial in seniors living in Beijing: Beijing Indoor Air Purifier StudY, BIAPSY. Sci Total Environ. 2017;603-604:541-549.  
  1. Chen R, Zhao A, Chen H, et al. Cardiopulmonary benefits of reducing indoor particles of outdoor origin: a randomized, double-blind crossover trial of air purifiers. J Am Coll Cardiol. 2015;65(21):2279-2287.  
  1. Karottki DG, Spilak M, Frederiksen M, et al. An indoor air filtration study in homes of elderly: cardiovascular and respiratory effects of exposure to particulate matter. Environ Health. 2013;12:116.  
  1. Kajbafzadeh M, Brauer M, Karlen B, et al. The impacts of traffic-related and woodsmoke particulate matter on measures of cardiovascular health: a HEPA filter intervention study. Occup Environ Med. 2015;72(6):394-400.  
  1. Maestas MM, Brook RD, Ziemba RA, et al. Reduction of personal PM2.5 exposure via indoor air filtration systems in Detroit: an intervention study. J Expo Sci Environ Epidemiol. 2019;29(4):484-490.  
  1. The World Bank. PM2.5 air pollution, mean annual exposure (micrograms per cubic meter) – United States. 2017. Available at: Accessed June 3, 2021. 
  1. United States Environmental Protection Agency. Particulate Matter (PM2.5) Trends. Last updated May 21, 2021. EPA Web site. Accessed June 3, 2021. 
  1. Dröge J, Müller R, Scutaru C, et al. Mobile Measurements of Particulate Matter in a Car Cabin: Local Variations, Contrasting Data from Mobile versus Stationary Measurements and the Effect of an Opened versus a Closed Window. Int J Environ Res Public Health. 2018;15(12):2642.  
  1. Vande Hey JD, Sonderfeld H, Jeanjean APR, et al. Experimental and modeling assessment of a novel automotive cabin PM2.5 removal system. Aerosol Sci Technol. 2018;52(11):1249-1265. Available at: Accessed June 4, 2021.   

Jacob Schor,ND, FABNO graduated from NCNM in 1991 and has practiced in Denver, CO, ever since. He has been active in state association politics, taking his turn as president of the Colorado Association of Naturopathic Doctors and Legislative Chair. Dr Schor has also held leadership positions in the Oncology Association of Naturopathic Physicians, served on the AANP Board of Directors, and chaired the AANP’s speaker selection committee. For the past decade he has been the Associate Editor of the Natural Medicine Journal, and is a regular contributor to the Townsend Letter.  

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