Vehicular pollutants can be felt and even seen as haze and smog when walking along the sidewalk. This ready visibility encourages an awareness of outdoor air pollution. For decades, research has focused on the air quality outside of vehicles. Research is beginning to indicate that the air inside vehicle interiors can be a serious health concern, for causes ranging from elevated heart rate and blood pressure to increased risk of driver drowsiness.

Globally, time spent on the road is increasing. There were 237 million passenger cars on roads in the European Union in 2018.¹ Prior to travel restrictions associated with the COVID-19 pandemic, monthly road and street travel in the United States in January 2020 was estimated to be 253 billion vehicle miles, up 5.3 billion vehicle miles from January 2019.²

Though the COVID-19 pandemic may have slowed traffic down in some parts of the world in 2020, congestion returned to the Indian cities to Mumbai, Bengaluru, Delhi, and Pune in early 2021.³ Worsening traffic is likely to continue as the pandemic subsides, increasing concentrations of both vehicle interior and outdoor pollution.

Why is the air inside a car worse than outside?

Pollutant levels are often higher inside vehicles because cars take in emissions from surrounding vehicles and recirculate them. Because vehicles are not built to be airtight, pollutants enter the car cabin through air vents and other openings.⁴

Roadway concentrations of vehicle-related pollutants are typically several times higher than ambient (outdoor) concentrations. Levels of some pollutants and toxic compounds can be as much as nine to twelve times higher inside vehicles than alongside the road.⁵

Furthermore, according to a 2016 study published in Environmental Science: Processes & Impacts, air pollution concentrations at the centerline of the road are highest and can be several times higher again than concentrations on the side of the road. It is these centerline concentrations that reflect the air getting into vehicles.⁶

What’s in vehicle exhaust?

Vehicle exhaust is a cocktail of dangerous pollutants, including:

• nitrogen dioxide (NO₂):Harsh-smelling gas that can cause breathing problems, lung irritation, and lower resistance to respiratory infections.⁷
• ozone (O₃): At ground level, ozone is toxic. When vehicles emit nitrogen oxides (NO), sunlight acts on them to create ground-level ozone. Ozone is the primary component of smog and is a major lung irritant. Long-term exposure may cause asthma and can result in serious and permanent structural damage to the lungs.⁸
• carbon monoxide (CO): Colorless, odorless, poisonous gas that causes short-term effects similar to oxygen deprivation, such as dizziness, fatigue, and confusion.⁹
• sulfur dioxide (SO₂): When this gas is inhaled, it can cause shortness of breath and chest pain. In the long term, it can cause acute respiratory illness and permanent changes to the lungs.¹⁰
• fine particles (PM2.5): Particles ranging in diameter between 0.1 and 2.5 microns. For comparison, the diameter of a single human hair ranges from 17 to 181 microns. When inhaled, these particles can lodge in lung tissue, triggering respiratory illnesses such as asthma, bronchitis, and emphysema.¹¹
  
  PM2.5 exposure has been linked to increasing the potential for cardiovascular problems, such as arrhythmic heartbeats and heart attacks. PM2.5 represent about 9 percent of all airborne particles. 10.2 million deaths in 2012, while, an estimated 160,000 million deaths in the world’s largest cities in 2020 were from PM2.5¹²
• ultrafine particles (UFPs): Particles smaller than 0.1 microns in diameter. About 90 percent of all airborne particles are this size. Not only are UFPs the most numerous of airborne particles, but they are also the most dangerous to your health.
  
  The tiny size of ultrafine particles enables them to be easily inhaled, deposited into the lungs, and absorbed directly into the bloodstream. From there, they travel with your bloodstream to all vital organs, including your brain. It’s estimated that the 6 percent of our days we spend in our cars can result in more than half of our UFP exposure.
• volatile organic compounds (VOCs): Chemical compounds emitted as gases, including acetone, benzene, formaldehyde, styrene, and xylene. While VOCs can be emitted from tailpipes, they can also be produced through chemical off-gassing in car interiors.

Intersections, red lights, and stop signs

Research published in the journal Atmospheric Environment measured air pollutants inside and outside vehicles at traffic intersections in urban and suburban areas. Their findings include:¹⁴

• stopping at red lights greatly increases exposure to air pollution
• intersections with traffic signals have up to 29 times higher concentrations of particulate matter than open roads
• drivers spent 2 percent of their time passing through intersections, which accounted for 25 percent of their pollution exposure

Air pollution levels are high at intersections with traffic lights because drivers decelerate, idle, and accelerate there. The same result will occur anywhere cars idle, such as drive-through windows at restaurants.

Heavy traffic means heavy pollution

Pollution inside cars stuck in heavy traffic or at a red traffic light is as much as 40 percent higher than when the car is moving.

The study previously mentioned in Atmospheric Environment found that when windows are closed and the fan is on inside the car, dirty air from outside the vehicle is brought in and sharply increases poor air quality.

Keeping the windows closed with the fan on (which brings in outside air) also significantly increased in-car pollutants. A 2021 study published in Science of the Total Environment measured particulate matter in cars within 10 global cities. Particulate matter exposure was highest when windows were open, especially during peak-morning hours.¹⁵ The majority of particle pollutants were PM10 when windows were open, while PM2.5 was more common when fans were on or when recirculating vehicle air.

How to improve in-car air quality

• Use a car air purifier. The portable Atem Car air cleaner uses HyperHEPA Plus particle filtration technology with gas-phase media to trap ultrafine particles and capture airborne pollutants from car interiors.
• Keep a safe distance from vehicles ahead of you, especially diesel trucks.
• When you’re in traffic or at a stop sign or light, close your windows, keep some distance from the vehicle in front of you, and put your air on recirculate.
• Try to use less congested roads with fewer traffic lights, even if they take a little longer. Try to avoid rush-hour traffic on busy roads or highways during peak hours.
• Don’t rely solely on an in-car air filtration system. They are not typically highly efficient at removing ultrafine particulates, VOCs, carbon monoxide, or other pollutants. Also, they don’t filter air entering the car cabin through windows and door cracks.
• Avoid car air fresheners or deodorizers. They are full of harmful VOCs and make your air quality worse – not better.
• Keep your car interior clean. Pollutants in cars can combine with dust particles and be inhaled. However, avoid chemical cleaners. Use a microfiber rag instead.

The takeaway

Our reliance on cars is likely to continue, but there are steps that can be taken to minimize our exposure to traffic pollutants. Following these easy steps can help minimize exposure to airborne pollutants in the car and help provide a safer breathing space during a daily commute.