Why indoor air pollution can be just as deadly as wildfire smoke and coal plant smog
Household air pollution is a hidden source of death and illness and climate change seems to be making it worse
Picturing air pollution isn’t hard in a world where 99% of people breathe polluted air. While reading this, you’re probably mentally visualizing an outdoor urban area with factories leaking out dense, dark smoke or intense car traffic releasing dirty fuels in rush hour. But although our immediate association of pollution tends to lean on the outdoors, the issue goes way beyond external environments.
Household air pollution is a silent and often neglected threat that causes about 3.2 million deaths in the world every year, according to World Health Organization (WHO) estimates. That corresponds to nearly half of the 6.7 million annual deaths worldwide attributable to air pollution. Among all lethal illnesses associated with household air pollution exposure, the organization points out that ischaemic heart disease accounts for 32% of the 3.2 million annual deaths, followed by stroke, pneumonia, chronic obstructive pulmonary disease and lung cancer.
Home is supposed to be a safe place. But people might not be aware they don’t even need to go outside to be exposed to harmful pollutants. The air inside can be just as dangerous.
Indoor air pollution can derive from a wide range of particles and components that people might be exposed to in and around their homes. It can also encompass other indoor environments where many people spend significant time, such as schools and workplaces. Among indoor pollution sources, those with the largest burden of disease globally are the ones associated with household fuel combustion from the use of dirty cooking, heating and lighting systems.
Current estimates from the WHO indicate that over 2 billion people — or around a third of the world's population — are exposed to household air pollution. Most of the affected population lives in low and middle-income countries, where many people still rely on polluting fuels and devices, especially for cooking. The most affected countries are in the global south including South Sudan, Burundi, Liberia, Uganda, Sierra Leone, Laos and Vanuatu. These are regions in which more than 90% of the population relies on these systems, according to WHO's Global Health Observatory data.
The problem lies in cooking with open fires or stoves fueled by kerosene, coal and biomass (such as wood, crop waste, and animal dung). These sources emit large amounts of pollutants including carbon monoxide, fine particulate matter and nitrogen oxide. 
Current estimates from the WHO indicate that over 2 billion people are exposed to household air pollution.
“These levels of pollutants from fuel combustion are very high, much more than you get necessarily in your ambient, outdoor air. It's typically in concentrations higher than that,” explained Heather Adair-Rohani, head of air quality, energy and health at the World Health Organization. But getting people to transition to alternative, safer ways of cooking isn’t easy. “A behavior change is a very big challenge to try and get households to uptake,” she said. “They've been doing it for so many generations, and this is what they're used to.”
Tackling polluting cooking systems, however, should not obliterate people’s caution with other important sources of air pollution like household heating, Heather notes. 
“In many cases, people are trying to solve the household air pollution problem by only looking at the cook stove, but if you still have the open fire burning to stay warm at night, this is mitigating much of the impact you may have had from the clean cooking aspects,” she said, adding that inefficient lighting such as kerosene lamps make up another significant source of household air pollution. “People typically are close to the kerosene lamp as well, causing them to really inhale a lot of the particles.”
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Fine particles, tobacco smoke and radon are among the most common pollutants in the U.S.
Although the use of polluting fuels for cooking is not as common in higher-income nations like the U.S., people may still be exposed to numerous other sources of air pollution in their homes.
“Indoor environmental sources and their impacts on human health and the indoor environment are highly variable due to many factors including difference in construction, location, individual sensitivities and occupant behavior,” a spokesperson for the U.S. Environmental Protection Agency (EPA) told Salon in an email. “With that qualification, we can point to some more common sources of concern [in the country]. Estimates of the health burden of fine particles (PM2.5) are consistently high, therefore exposure to sources of fine particles, such as outdoor air (including wildfires), poorly-vented indoor combustion (e.g., from cooking and heating), and tobacco smoking can be of concern for large parts of the population.”
After tobacco smoking, radon is the country’s second leading cause of lung cancer, causing 21,000 deaths every year.
The EPA also notes many other chemical solvents used indoors can be sources of toxic aldehydes and other volatile organic compounds, which have a wide range of health effects that may vary from eye and airway irritation to the impairment of vital body parts such as the central nervous system, liver and kidney. The agency warned that formaldehyde, and other contaminants, which can be found in building materials  such as composite wood products, can be toxic or react with ozone or other pollutants to form toxic substances. Finally, it highlighted the risk of biological contaminants, including molds and infectious viral and bacterial agents.
Another threat that affects indoor environments in the U.S. and around the world is radon gas infiltration, which “continues to be a well-characterized indoor air risk in many regions of the country,” the EPA said, which has developed a map of areas in the country with a potential range for elevated indoor radon levels. It is a radioactive, colorless and odorless gas that can be found in the soil, bedrock and groundwater, getting into homes through cracks and cavities in the building, as well as through groundwater supply.
After tobacco smoking, radon is the country’s second leading cause of lung cancer, causing 21,000 deaths every year. For people who smoke and are exposed to radon, the risk of developing lung cancer multiplies to around 10 times greater compared to those who don’t use tobacco. 
To improve air quality indoors, the EPA recommends the first priority is controlling polluting sources inside our homes.
“Improving ventilation is a general-purpose strategy that can be effective for many indoor air quality problems, provided the quality of outdoor air is good,” the agency informed. “The treatment of indoor air with supplemental filtration-based portable air cleaners or improved HVAC filters (rated MERV 13 or higher, when possible) is a third option that is effective for particle pollution.”
The agency recommends that any household test radon levels, which can be done with a relatively inexpensive test that measures indoor concentrations of the gas.
The smaller, the more dangerous to our health
A grain of sand is generally used for expressing tininess, or insignificance. When we bring the subject to the scope of air pollution, though, tiny particles can grow in significance to human health the smaller they get. While beach sand is around 90 micrometers in diameter, and human hair around 50 to 70 micrometers, the particles that are 10 micrometers or less are those posing the greatest risks to our health. These very small particles suspended in the air make up particulate matter, which, like carbon monoxide, is a major pollutant found in household fuel combustion.
The larger, so-called coarse particles, have diameters between 2.5 and 10 micrometers; some examples of this type of particulate matter include wind-blown dust, fly ash and animal or vegetal particles. These are typically stopped in our thoracic cavity as they enter the lungs.
Children, women and elderly people are generally more vulnerable to the risks of indoor air pollution.
The reason the smaller group of airborne particles, referred to as fine particles and with 2.5 micrometers or less in size, can be so much more dangerous to human health is because they can pass through the lungs and enter the bloodstream, affecting vital body parts such as the heart and the brain. These particles are commonly found in smoke, soot or haze.“It's so small that it can get in and really impact the body systemically,” Adair-Rohani said.
Children, women and elderly people are generally more vulnerable to the risks of indoor air pollution. Aside from typically spending more time indoors, the elderly population can also be at increased risk due to the higher prevalence of other diseases that exacerbate air pollution effects, the EPA notes. The WHO points out that children and women around the world tend to spend more time exposed to polluting sources due to their daily routines and consequent exposure to dirty fuels and devices.
There are also physical aspects involved in air pollution health risks: “Children are considered a vulnerable population for air pollution (indoor or outdoor) because of developing respiratory systems and metabolic factors,” the EPA explained. According to global WHO estimates, every year over 237,000 deaths of children under the age of five are associated with household air pollution. Health effects might originate even before birth: researchers have pointed out correlations between prenatal exposure to high levels of air pollution and impacts on neurodevelopment in early childhood. There are studies that also suggest that air pollution can impact women’s reproductive health.
Climate change might aggravate air pollution levels, including indoors
Climate change, which is driven by burning fossil fuels that emit greenhouse gasses, is expected to make both indoor and outdoor air pollution worse. As these emissions continue to rise, extreme events like wildfires are projected to increase in frequency and intensity. Experts point out that global warming effects might impact outdoor and indoor air pollution in several ways primarily due to the inevitable exchange between outdoor and indoor air.
“Climate change is known to increase ground-level ozone and particulate matter, and lead to increases in wildfire smoke, an increase in dust in the southwest (as well as dust-borne pathogens like valley fever), and increases in some kinds of aero-allergens,” the EPA said. “These increases in outdoor air pollution will also make indoor air pollution worse.”
Carbon monoxide concentrations can also increase indoors due to climate change effects.
The impacts can typically go the inverse way as well, as indoor emissions also contribute to increasing outdoor air pollution. “In some states of India, over 50% of ambient air pollution is actually caused by the household air pollution, leaking outdoors,” Adair-Rohani exemplified. “Pollution is being generated in the home, it's going in the chimney, but then it goes into the local communities.”
Another important factor is that household air pollution is one of the largest sources of black carbon, which has a high warming potential, she notes. An estimated 25%  to 50% of global black carbon emissions come from residential fuel combustion. It is a short-lived pollutant that only lasts a few days or weeks in the atmosphere, but is one of the largest contributors to climate change since its warming impact is estimated to be up to 1,500 times stronger than CO2.
Carbon monoxide concentrations can also increase indoors due to climate change effects, the EPA explained, given an increase in power outages from storms and other extreme events can lead to HVAC system interruptions, encouraging the use of generators. Also, the agency notes that as climate change will increase total precipitation in the east, it can also enhance the impacts of biological contaminants such as mold, fungus and bacteria in indoor spaces. Furthermore, it cites that climate effects may change people’s relative exposure to indoor versus outdoor air. “Increases in temperature will lead to changes in behavior: more time spent indoors with windows closed in the summer, but less time in other seasons.”
Improving indoor air quality through realistic energy transition
At the community level, reducing indoor air pollution requires that nations transition to cleaner energy sources. Adair-Rohaniobserves, though, that this transition may vary according to the country or region's status regarding the use of clean energy. “One high-income country may have a problem with natural gas in the home because of asthma, so they could lead to electric renewable cooking; that would be the ultimate clean one,” she said. “Whereas households in some African countries, for example, where 99% of their population are relying on traditional fuels and stoves for cooking, that's a whole different problem.”
Adair-Rohani adds that tackling household air pollution is also an important way to help mitigate the effects of climate change, but acknowledges that access to cleaner systems continues to be a challenge for many populations.
“In some cases, buying cleaner fuels and technology to use requires some upfront investment,” Adair-Rohani said. That is why she believes it’s critical that countries provide people with “an enabling policy framework and situation where they can actually get access to these fuels and technology, that they're available and affordable, and that they meet the needs and preferences of the users.”
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