Air pollution, a silent but lethal adversary, pervades our modern world, casting a shadow over human health and the environment. Defined by the World Health Organization (WHO) as the “contamination of the indoor or outdoor environment by any chemical, physical or biological agent that modifies the natural characteristics of the atmosphere,” air pollution poses a grave threat to public health, the climate, and the earth’s ecosystem.
The 19th-century invention of the internal combustion engine, which revolutionized transportation, has posed, and continues to pose a particularly significant environmental threat. This type of engine operates through the ignition and combustion of a fuel-air mixture within a confined space. The resulting expansion of gases drives a piston, which ultimately produces mechanical power.
Internal combustion engines come in different forms: reciprocating or rotary, spark or compression ignition, and two- or four-stroke configurations. Additionally, they differ in whether they use diesel or gasoline. The most common combination is the reciprocating, spark-ignited, four-stroke gasoline engine found in the majority of automobiles. According to Theodore Gray in his book Engine: The Inner Workings of Machines That Move the World, “The four-stroke engine separates each of the four tasks necessary to make a complete cycle: 1) draw in fresh fuel-air mix, 2) compress it, 3) ignite and expand it, then 4) flush out the exhaust.” This design ensures efficient combustion and a process for treating the exhaust, thereby minimizing pollution.
Two-stroke engines, instead of separating all four tasks, utilize a simplified operational cycle. Essentially, they combine the compression and ignition phases in one stroke and the intake and the exhaust phases in another. This simpler design, however, comes at the cost of lower fuel efficiency and greater exhaust emissions: large amounts of soot and unburned fuel contaminate the environment and expose the operator to dangerous levels of pollutants. These engines cause more environmental contamination for two primary reasons. First, because the intake and exhaust strokes occur simultaneously, unburnt fuel inevitably escapes from the exhaust, and the two-stroke engines lack catalytic converters to treat the emissions. Second, these engines require oil, which is mixed with the fuel to lubricate the engine’s moving parts. This oil helps create additional pollutants during the combustion process: higher emissions of hydrocarbons like benzene, which has been associated with adverse health effects, and of particulate matter (PM). Additionally, combustion of the fuel produces sulfur dioxide and sulfur trioxide, which react with water to form sulfurous acid and sulfuric acid, respectively. These acids are harmful for breathing and damaging to the ecosystem.
Carbon monoxide is another by-product of these engines, produced in large amounts because of an inadequate fuel-air ratio, and it is toxic because it binds to hemoglobin in the blood, thereby inhibiting oxygen transport. Yet another issue relates to nitrogen oxides created by high-temperature combustion. These oxides are highly reactive and combine with ammonia and volatile organic compounds (VOCs) to form PM 2.5 , particulate matter of size 2.5 uM or less. PM 2.5 easily penetrates the lungs, causes respiratory diseases, and aggravates pre-existing diseases that can lead to premature death. A recent study in JAMA states that “probably no safe level of exposure to PM 2.5 exists—that is, the level below which no adverse health effects are observed.” Furthermore, when nitrogen oxides combine with sunlight, the reaction produces ozone. This ground-level ozone not only traps pollution, but it also specifically affects our lungs: breathing ozone-polluted air can cause coughing and shortness of breath, damage and inflame airways, and aggravate asthma.
Low-power engines—such as those in lawn and garden equipment, jet skis, snow mobiles, and mopeds—fall into the two-stroke category and emit disproportionately high levels of pollutants compared to their four-stroke counterparts. According to a study by Edmunds, a firm providing online information about the automotive industry, “hydrocarbon emissions from a half hour of yard work with the two-stroke leaf blower are about the same as [those emitted during] a 3900-mile drive from Texas to Alaska in a [2011] Ford F-150 pickup truck.” This alarming number puts the environmental issue into stark perspective.
Where does the pollution go? Once the pollutants are released into the atmosphere, their dispersion depends on various factors, including atmospheric conditions, the season, and geographical features. In cities surrounded by mountains, such as Mexico City and Salt Lake City, temperature inversions exacerbate air pollution, especially during the winter months. An inversion is a meteorological condition that is created when warmer air settles on top of a valley and traps colder air beneath it, along with pollutants emitted from various sources, mainly cars, lawn and garden equipment and industry. This stagnant layer of polluted air is often visible (referred to as haze) and requires mixing by a weather system before it can disperse.
During these elevated PM 2.5 inversion days, residents are cautioned to stay indoors to avoid exposure. This recommendation assumes that indoor air quality is better. However, this is not always true. According to the Environmental Protection Agency, indoor concentrations of “some pollutants are often 2 to 5 times higher than [the] typical outdoor concentration.”
Indoor air quality is often compromised by various sources, including combustion of oil, gas, kerosene, coal, wood, and candles; emissions of VOCs from building materials, new furniture or cabinetry; ultrasonic humidifiers; and household cleaning and personal care products. According to the WHO, indoor air pollution alone was responsible for an estimated 3.2 million deaths in 2020, including over 237,000 deaths of children under the age of 5. So simply advising people to stay indoors during poor air quality days does not amount to a reasonable solution; the truly sustainable solution, of course, is to prevent pollution to begin with.
While some people have recommended growing indoor houseplants to improve air quality, there is no evidence that plants are as effective as air purifiers with HEPA f ilters or activated carbon. Air purifiers, however, are not sustainable in the long term because of the ongoing costs of replacing the filters and the waste generated from using the purifiers (dirty filters go in landfills, and electricity must be generated to run them). The most efficient and simplest way to improve indoor air quality is through adequate ventilation. Opening windows and doors can improve indoor air quality dramatically if the outdoor air is clean.
In conclusion, air pollution presents a significant threat not only to public health but also to environmental sustainability. Concerted efforts must be made both at the individual and the governmental levels to address both indoor and outdoor air quality. Addressing air pollution requires multifaceted strategies, some of which are well known.
What can government do? To improve outdoor air, governmental organizations must provide and promote effective public transportation systems, incentivize carpooling, biking, and walking. While car emissions are recognized to be large contributors to this problem, pollution from two-stroke engines must not be overlooked. These engines should be phased out in exchange for cleaner alternatives. Air quality should be prioritized now. Inspiration can be drawn from successful initiatives implemented in other parts of the world, for instance, Mexico City’s Hoy no Circula program or Manilla’s UVVRP program. Similar measures can be adapted in cities facing air pollution challenges. For improved indoor air quality, adequate ventilation must be designed into buildings as opposed to airtight ones where carbon dioxide and pollutants accumulate inevitably. Furthermore, materials used in construction including glues for cabinetry that emit VOCs like formaldehyde must be banned. Household cleaning products and personal hygiene products should be subjected to higher regulatory standards.
What can individuals do? For outdoor air, individuals should spend less time driving their own cars, carpool, take public transportation where there are options, and consider alternative modes of travel, such as cycling and walking when possible. For indoor air, it is crucial to re-evaluate the romanticizing of activities like candle burning and using gas or wood fireplaces as these contribute to indoor air pollution. Cooking especially frying and grilling, produces a substantial amount of PM 2.5 and other pollutants such as polycyclic aromatic hydrocarbons, formaldehyde, and acetaldehyde. Since cooking is necessary for living, our best options to reduce exposure is to monitor indoor air quality and ventilate adequately. It is evident that outdoor and indoor air quality are interconnected; our well-being, ultimately, is tied to outdoor air.
Editor’s note: The opinions in this article are my own and not the views of the Salt Lake City’s Department of Economic Development.
Atim Enyenihi, PhD, has spent the last decade living and working in Salt Lake City, where air quality issues are paramount as a result of the area’s geography. She is devoted to educating the public and championing solutions that improve both indoor and outdoor air quality for a more sustainable planet and improved human health. Enyenihi earned her doctorate in chemistry from the University of North Carolina at Chapel Hill and her post doc from the Karolinska Institute in Sweden. Her career has spanned academia, biotechnology, in-vitro diagnostics, the pharmaceutical industry, and government.
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