Cars: What Pollutants Are They Emitting?

Cars, trucks, and buses produce air pollution throughout their life cycle, including pollution emitted during vehicle operation and fuel production. The transportation sector is a major source of air pollution, contributing to climate change and causing serious health issues. Vehicle emissions contain harmful pollutants such as particulate matter, volatile organic compounds (VOCs), nitrogen oxides (NOx), carbon monoxide (CO), and sulfur dioxide (SO2). These pollutants have detrimental effects on human health, the environment, and global warming. Understanding and addressing the impact of car pollution are crucial steps towards improving air quality and safeguarding public health.

Carbon monoxide (CO)

Cars emit carbon monoxide when fuel is burned, and the Environmental Protection Agency estimates that vehicles cause nearly 75% of carbon monoxide pollution in the United States. According to the agency, up to 95% of all CO emissions in cities may come from motor vehicle exhaust. Breathing air with a high concentration of CO is extremely dangerous as it affects critical organs like the heart and brain by blocking oxygen from reaching them. Prolonged exposure to carbon monoxide reduces the blood's ability to carry oxygen to the body's organs, leading to symptoms such as throbbing headaches, disorientation, drowsiness, convulsions, unconsciousness, and eventually death.

Carbon monoxide poisoning from vehicles can occur in various scenarios. One of the most common is when a vehicle is left running in an enclosed space, such as a garage, even with the garage door open. This is because carbon monoxide can accumulate to dangerous levels, and the catalytic converter, which converts carbon monoxide to non-toxic CO2, is ineffective in low-oxygen environments. Operating a vehicle with a defective exhaust system or driving with the trunk lid or rear tailgate open can also increase the risk of carbon monoxide exposure.

Additionally, certain vehicle malfunctions, such as a broken or leaking exhaust system, can lead to carbon monoxide leaks into the cabin of the car. While these malfunctions often produce loud noises or noticeable smells, it is crucial to maintain and inspect vehicles regularly to identify potential issues. Carbon monoxide detectors and alarms specifically designed for vehicles can also help identify the presence of this toxic gas and prevent poisoning.

Furthermore, changes in engine design, fuel, and emission control devices have contributed to reducing CO emissions from vehicles. However, technological advancements, such as the introduction of keyless ignition vehicles, have also introduced new risks. For example, without the physical act of turning off the ignition and removing the key, some owners inadvertently leave the engine running, potentially leading to carbon monoxide buildup.

Nitrogen oxides (NOx)

NOx is a significant contributor to air pollution, particularly in areas with high motor vehicle traffic, such as large cities. The transportation sector, including cars, trucks, and buses, is responsible for over 55% of NOx emissions in the United States. These emissions have severe consequences for air quality and public health.

Nitrogen oxides irritate the respiratory system, causing coughing and choking, and reducing lung capacity. They weaken the body's defences against respiratory infections such as pneumonia and influenza. Additionally, NOx plays a crucial role in the formation of smog and acid rain. When NOx combines with hydrocarbons in the presence of sunlight, they produce ground-level ozone, a primary component of smog. This ground-level ozone contributes to respiratory problems and has detrimental effects on human health.

Furthermore, NOx is involved in ozone depletion in the upper atmosphere, leading to an increased risk of ultraviolet radiation reaching the Earth's surface. While ozone depletion occurs naturally due to lightning, human activities, such as vehicle emissions, significantly contribute to this issue.

It is important to note that NOx emissions are not limited to vehicles. Agricultural practices, such as the use of nitrogen-fixing plants and nitrogen fertilizers, also contribute to NOx levels in the atmosphere. This has been observed in various states, including California, where nitrogen fertilization is a significant source of NOx emissions.

Volatile Organic Compounds (VOCs)

Oxygenated volatile organic compounds (OVOCs) are a subclass of VOCs that are of particular concern, as they account for a significant proportion of total VOC emissions from diesel vehicles. OVOCs have been found to contribute between 9.4% and 52% of total VOC emissions for gasoline vehicles, with significantly higher fractions for diesel vehicles, ranging from 52% to 71%.

The emission factors of VOCs are influenced by various factors, including engine activities, driving conditions, vehicle model, driving speed, air exchange rate, temperature, and the types of substances with different boiling points inside the vehicles. For example, cold starts have been shown to significantly influence the VOC emissions of gasoline vehicles, while this factor is less important for diesel vehicles.

The concentration of VOCs inside vehicles can also be affected by temperature, with higher temperatures leading to increased concentrations of VOCs. Additionally, the interior materials of vehicles can release VOCs, including alkanes, alcohols, ketones, benzenes, alkenes, aldehydes, esters, and naphthalene.

Reducing VOC emissions from vehicles can be achieved through various means, such as tightening emission standards, reducing traffic congestion, and improving vehicle efficiency. Driving habits, such as observing speed limits, accelerating gradually, and maintaining a steady speed, can also help lower VOC emissions.

Particulate matter (PM)

PM can be a primary pollutant or a secondary pollutant from hydrocarbons, nitrogen oxides, and sulfur dioxides. Diesel exhaust is a major contributor to PM pollution. In addition to diesel, the burning of sulfur-containing fuels, especially coal, is another source of particulate matter. Sulfur dioxide can react in the atmosphere to form fine particles, which pose the largest health risk to young children and asthmatics.

Traffic-related PM has been associated with impaired cognitive functions and an increased incidence of neurodegenerative diseases such as Alzheimer's disease. Inhaled components of PM may directly invade the brain via the olfactory route or act through peripheral system responses, resulting in inflammation and oxidative stress in the brain. Large-scale epidemiological studies have also linked PM exposure to an increased risk of dementia, Parkinson's disease, and multiple sclerosis.

In-car PM exposure has been measured in several global cities, with windows-open scenarios resulting in the highest PM concentrations. PM exposure was also found to be significantly higher during morning peak hours. Recirculation and off-peak trips showed lower PM exposure, with up to an 80% reduction in PM2.5 levels compared to windows-open conditions.

While transportation, including cars, trucks, and buses, is a significant contributor to air pollution, it is important to note that many other human activities also play a role. The way people drive, such as observing speed limits and accelerating gradually, can also influence the amount of pollution emitted from their vehicles.

Greenhouse gases

According to the US Department of Energy, highway vehicles release about 1.7 billion tons of greenhouse gases into the atmosphere each year. Each gallon of gasoline burned creates 20 pounds of greenhouse gases, or roughly 6 to 9 tons per year for a typical vehicle. The amount of carbon dioxide emitted is proportional to fuel consumption and inversely proportional to fuel economy—a 1% increase in fuel consumption results in a 1% increase in carbon dioxide emissions. A typical passenger vehicle emits about 4.6 metric tons of carbon dioxide per year, though this varies based on the vehicle's fuel, fuel economy, and the number of miles driven per year.

In addition to the combustion of gasoline, the production and distribution of gasoline also create greenhouse gas emissions. This includes extracting oil from the ground, transporting it to a refinery, refining the oil into gasoline, and transporting the gasoline to service stations. Electric vehicles do not emit tailpipe emissions, but emissions are created during the production and distribution of the electricity used to fuel the vehicle.

Transport is the only sector where greenhouse gas emissions have increased in the past three decades, rising 33.5% between 1990 and 2019. Passenger cars account for 61% of total CO2 emissions from EU road transport. Strategies to reduce greenhouse gas emissions from transportation include low-carbon fuels, new and improved vehicle technologies, reducing the number of vehicle miles travelled, and operating vehicles more efficiently.

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