Health Letter, September 2023
By Nina Zeldes, MSc, Ph.D.
The recent wildfires in North America have led to several U.S. cities suffering from some of the worst air quality levels in the world, putting millions of Americans under air quality alerts. However, according to the World Health Organization (WHO), almost all of the world’s population is exposed to air pollutants that exceed air quality guidelines. This has serious consequences because both short- and long-term exposure to poor air quality is a significant contributor to many health conditions, including asthma, heart disease and dementia, and contributes to millions of deaths annually.
What is air pollution?
Air pollution is a contamination of indoor or outdoor air by chemical, biological or physical substances, some of which are so small that they may be inhaled deep into the lungs, enter the bloodstream and end up in different organs. They pose a danger to human health but also harm animals and plants, damage materials (such as buildings) and are a major factor in the contamination of water and food. Most air pollutants are synthetic, although there are also some natural sources (such as wildfires or volcanic activity). These pollutants can be divided into four main groups: particulate matter, gases, persistent organic pollutants and heavy metals.
Particulate matter (PM) is a mix of solid particles and liquid droplets. It is one of the most harmful groups of pollutants and may be responsible for more than 60% of deaths attributable to environmental causes. The smallest particles, called PM10 and PM2.5 because they are no larger than 10 and 2.5 micrometers, respectively (the equivalent of less than 1/30th of the width of a human hair), are so small that they easily enter the human body. Research indicates that PM2.5 emitted by wildfires may be more harmful than particles from other sources, such as traffic.
Another key source of poor air quality are gases, many of which also interact with each other. They include sulfur dioxide (SO2), nitrogen oxide (NOx), nitrogen dioxide (NO2), carbon monoxide (CO), ground-level ozone (O3) and volatile organic compounds such as formaldehyde.
Some persistent organic pollutants, such as certain pesticides or dioxins, and heavy metals, including lead and mercury, also contribute to air pollution. They can persist in the environment for long periods and accumulate in human bodies over time.
Key sources of outdoor and indoor pollution
One of the biggest contributors to outdoor air pollution are emissions from fossil fuels (oil, coal or gas). However, there are many sources of poor air quality, including motor vehicles, power plants, factories, construction activities, waste incineration, agriculture and wildfires. In recent years, outdoor air pollution guidelines have helped to reduce the emissions from some of these sources; however, according to the WHO, outdoor air pollution still causes more than 3 million deaths a year.
Indoor pollution kills almost as many people annually, but there are no recent guidelines for indoor air quality or air quality controls. That is of particular concern because people tend to spend most of their time indoors and, according to the Centers for Disease Control and Prevention, pollutants can more easily reach dangerously high concentrations indoors, especially without adequate ventilation.
The sources of indoor pollution tend to be more diverse than those for outdoor pollutants, although of course outdoor pollutants also can enter indoor spaces, for example when windows are opened to increase ventilation. The composition of certain indoor pollutants also differs from that of their outdoor equivalents. For example, one of the main sources for PM2.5 outdoors is traffic, whereas indoors, cooking appliances are a main source. Cooking appliances, such as gas ranges, also release several other air pollutants, some of which, such as methane or nitrogen oxide, are even released when the appliances are turned off. These emissions have been linked to higher rates of childhood asthma.
In addition to gas ranges, sources for poor indoor air quality include other devices for heating or cooking (such as fireplaces or woodstoves), furniture and flooring made from pressed wood, carpets and upholstery, paint, cleaning products, air fresheners and personal care products such as cosmetics.
Other airborne pollutants that are of particular concern in closed spaces include tobacco smoke, mildew, fungal spores from molds and — as has been discussed throughout the COVID-19 pandemic — certain viruses and bacteria.
The most common health effects of air pollution
Some pollutants cause immediate reactions, such as irritation of the eyes, nose, throat or skin, as well as nausea, dizziness or headaches. In some cases, these can be addressed by removing or avoiding the source of the irritation. Pollutants also can aggravate existing conditions or lead to severe diseases including cancer, many of which are only diagnosed a long time after exposure. However, both short- and long-term exposure to poor air quality can lead to serious and life-threatening health outcomes, including the following:
- Reduced life expectancy and premature death
Poor air quality is associated with more hospital admissions, and premature death.
- Cardiovascular disease
Many of these deaths are due to cardiovascular diseases. Numerous studies have shown that exposure to air pollution, including wildfires, increases the risk of myocardial infarction, cardiac arrhythmias, ischemic heart disease and heart failure. This risk also is present at air pollution levels that are below many current guidelines. Short- and long-term exposure to air pollution also has been associated with an increased risk of stroke.
- Respiratory and lung diseases
Because air pollutants irritate the airways and lungs, they can cause difficulty breathing, reduce lung function and increase susceptibility to respiratory infections. They also can cause or exacerbate asthma, chronic bronchitis, or chronic obstructive pulmonary disease (COPD). Long-term exposure to air pollution can even lead to lung cancer.
- Cognitive decline and neurodegenerative diseases
Poor air quality can lead to temporary reductions in the ability to concentrate and can cause slowed reflexes and confusion. Long-term exposure, and especially exposure to PM2.5, is associated with an increased risk of cognitive decline and neurodegenerative diseases, such as Alzheimer’s disease, dementia and Parkinson’s disease. This risk seems to be present even when exposure to PM2.5 is below current guidelines. Moreover, research indicates that some sources of PM2.5 appear to put older adults at a higher risk for developing dementia than others. For instance PM2.5 stemming from agriculture or wildfires seems to be more concerning than PM2.5 stemming from windblown dust or non-coal energy production.
- Mental health
Several studies indicate that air pollutants, and particularly long-term exposure to PM2.5, can be detrimental for mental health and, for example, increase the risk of depression and anxiety.
- Pregnancy and birth outcomes
Air pollutants can contribute to preterm birth, low birth weight and infant mortality.
Who is most at risk?
Poor air quality can be dangerous for everyone, but certain groups, such as pregnant people or those suffering from certain health conditions (including asthma, COPD or heart disease) are particularly at risk. Children, because their bodies and brains are still developing, are also at increased risk of short- and long-term health effects, and according to the WHO about 1 in 10 deaths in children under the age of 5 is caused by air pollution.
And although most regions are affected at least to some degree by poor air quality, low- and middle-income countries are at the highest risk from exposure to dangerously high levels of air pollution.
Within the United States, low-income and marginalized people are at a higher risk, in part because they are more likely to reside in areas with high outdoor pollution, live in buildings with poorer indoor air quality and already suffer from health conditions that make them more vulnerable to the dangerous health effects of air pollution. Furthermore, schools attended by low-income and minority students are more likely to be associated with poor air quality, which in turn has been linked to more missed school days and lower grade averages and test scores.
When addressing air pollution, the reduction of these environmental injustices needs to be a high priority, especially because some pollutants, such as PM2.5, pose a health risk even at levels that are below current guidelines.
How you can protect yourself
The best way to protect yourself from harmful air quality is work individually and collectively to reduce the amount of indoor and outdoor pollutants from being released in the first place. For example, a majority of deaths attributed to air pollution could be avoided if the WHO air quality recommendations were met. Moreover, because many of the key sources of air pollution are either also drivers of climate change (such as fossil fuel) or are exacerbated by climate change (such as wildfires), their reduction is of the utmost importance, not only to improve air quality but also for the health of our planet.
You can try to reduce your exposure by avoiding areas where a lot of pollutants are released, such as busy streets during peak hours. It can also be useful to monitor the level of air pollution in your area before engaging in outdoor activities, for example by using the U.S. government’s air quality index AirNow or by checking air quality alerts that are now available through many weather apps on smartphones. Most air quality measures color-code air quality levels (usually ranging from green to dark red) or use a scale that ranges from 0-500 to indicate the severity of pollution. According to this scale, no special precautions are necessary for air quality levels that are rated good (ranging from 0-50) or moderate (51-100). However, starting at 101-150, a reduction of outdoor activities, including exercise, is recommended for groups who are particularly sensitive to air pollution. Starting at 151-200, pollution levels may cause health effects for everyone, however especially among sensitive groups. Health warnings are in effect for everyone once the air quality is classified as very unhealthy (201-300) or hazardous (301-500). Vulnerable populations especially should consider wearing N95 face masks and installing indoor air-filtration systems. However, it is important that all efforts to reduce exposure are made equitably across countries and within communities.