Clean Air While Driving
Transportation environments pose a challenge to ensure safe air. Pollutants can originate from the car materials, from other passengers or outdoor air pollution. However, there are tools available to help. If they are used properly, travelling can be made much safer for everyone.
This post will first go through the main air pollutants that people are exposed to in cars. Then it will discuss the mitigation methods available.
Pollutants
Airborne Diseases
Ignoring air quality in cars can pose a significant threat of airborne disease transmission. The size of the space can play an important role in the risk of airborne disease transmission as smaller spaces fill up with pollutants quickly. Cars have a very small volume.
Even at the beginning of the COVID-19 pandemic, it was evident that transportation was a high-risk environment for airborne disease transmission. If windows are closed and the ventilation is off, there is very little outdoor air entering the car, leading to almost no air cleaning and a potentially very high concentration of infectious aerosols.
Carbon dioxide
There is no clear consensus on whether CO2 directly harms cognitive functioning. However, there is agreement that poorly ventilated areas have higher CO2 concentrations and can impair cognitive functioning, possibly from other pollutants. In other words, CO2 might not be a direct pollutant, but can serve as a helpful tracer gas.
Without ventilation, CO2 levels in cars can get very high fast — even exceeding 5000 ppm. This can possibly lead to drowsiness and cause driver impairment.
VOCs
The new car smell is not a good thing. It’s caused by volatile organic compounds (VOCs) which are the second most harmful air pollutant, after particulate matter. Two common ones that off-gas from car materials are benzene and formaldehyde which are both carcinogens.
There’s also strong evidence of very high VOC concentrations in cars sitting in the sun when it is hot outside.
Outdoor Air Pollution
Unfortunately, the tremendous amounts of indoor pollutants generated in cars are not the whole story. Busy roads can be very polluted. Motor vehicles exhaust contains all the most harmful air pollutants including particulate matter, VOCs and nitrogen dioxide. Often outdoor air can also be harmful with smog or wildfire smoke.
This means that ventilation isn’t always a perfect solution. Allowing in outdoor air can reduce the indoor air pollutants (airborne diseases, CO2, body odours and VOCs), but can bring in the outdoor air pollutants. Any solutions to optimize indoor air quality need to address both indoor and outdoor pollutants which can both be significant in transportation.
Tools Available
All these problems are solvable. We have the tools needed to improve air quality in cars. The main tools are windows and the climate control system. Small portable air cleaners for inside cars also exist, but as we will see, they are not a good solution.
Windows
Windows are the simplest and most effective tool available to provide ventilation. Research has found that when driving below 30 mph (50 km/h), having all windows open is optimal. When driving faster, having two windows open on opposite angles is better.
Even if the windows are not down all the way, having them cracked can still help. If one window is down while the vehicle is in motion, there can be an uncomfortable pulsating air pressure effect called buffeting. This can be stopped by opening a second window.
Opening windows can be challenging if there is outdoor air pollution, if it is cold outside or if the car is moving fast. In those conditions, the climate control system should be used.
Climate Control System
The climate control system can be an effective tool to control air quality in a car. It typically allows the driver to control:
- fan speed
- air temperature
- ventilation setting (feet, face or windshield defrost)
- recirculation mode
It also sometimes has an option to enable/disable the air conditioning. If the windows are open all the way, it isn’t necessary to use the climate control system because airflow from the windows can be significantly higher than what can be provided through the climate control system. If the windows are not fully open, it should be used.
The fan setting should be on the highest setting, noise permitting. The temperature should be set to keep you comfortable. If the cabin is comfortable, then set the air to supply at a neutral temperature, but keep the fan setting high. The ventilation setting (feet, face of windshield defrost) should not make any significant difference.
Filter
Many standard cabin filters are effective against fine particulate matter (PM2.5), although level of effectiveness can vary. Therefore, whether or not the car is in recirculation mode, it is still beneficial to have the fan running to filter the air.
Upgrading the cabin air filter to a HEPA filter is a beneficial, particularly for individuals with extended periods of car use, such as those with frequent long commutes or professional drivers for services like taxis, Lyft, or Uber.
Recirculation Mode
Recirculation mode prevents any outdoor air entering through the climate control system. Air will still circulate through the cabin filter when in recirculation mode.
Many cars default to recirculation mode on start-up and require the driver to turn it off.
When in recirculation mode, the filter can remove particles, like infectious aerosols. Gases like CO2, VOCs and body odours will not be removed and will increase in concentration. Unless the filter is a HEPA filter, it’s effectiveness against infectious aerosols might be limited. It will still likely help, but supplying outdoor air is probably more effective.
If outdoor air pollution is a problem, then the car should be put in recirculation mode. My general rule is I put it in recirculation mode if there is known bad outdoor air pollution (like wildfire smoke) or if I can smell something from outside, like the exhaust from other cars coming in.
Occasionally, I put the car in recirculation mode temporarily. This might be caused by a heavily polluting car passing on the road or being stopped at a light with the wind blowing a nearby cars exhaust directly at the intake. Once the situation passes, I turn off recirculation mode.
Most of the time, when outdoor air pollution is not a clear problem, the recirculation mode should be taken off and the car should be bringing in outdoor air. I find it is not difficult to keep the car CO2 concentrations at around 600–700 ppm with just the climate control system alone.
Advanced Climate Controls & Auto Mode
Some models have advanced climate controls. In general, these have similar capabilities as standard climate controls with one important exception — auto mode. Auto mode has preprogrammed settings to control temperature, fan speed, ventilation mode and recirculation mode and adjusts these based on car temperature.
With all air quality controls equipment, auto mode is generally a very bad idea. It is a way of saving energy by lowering the clean air delivery rate:
- It is used on thermostats for ventilation equipment to stop ventilation when the space temperature is satisfied.
- It is used on portable air cleaners to reduce speed when particulate matter is low.
- It is used in cars to reduce fan speed and override recirculation mode on when the temperature is satisfied.
Auto mode should more appropriately be called “reduced air quality mode”.
Even if recirculation mode is off, some car models recirculate air in auto mode.
Make sure auto mode is off.
Flushing
When you first enter your car, especially if it has been sitting in the sun, leave the doors open for a few seconds, keep your windows open for a minute and turn the ventilation on all the way, not in recirculation mode. Since the space is tiny, flushing will be completed very quickly. This process does not take much time and can significantly reduce your exposure to the build up of VOCs when you first enter the car.
Portable Air Cleaners
There are many tiny portable air cleaners that are marketed for cars. This is because the car has a very small volume, so a smaller air cleaner can still clean a larger percentage of the air faster.
However, this premise is flawed. What is ultimately important is the clean air delivery rate. The CADR from the climate control system can be around 100–400 cubic feet per minute (CFM) of outdoor or filtered air. The CADR from open windows can be even greater, depending on the car speed or wind. The CADR from portable air cleaners is often 10–30 CFM — an order of magnitude lower than other measures.
One of the main harms of specifying air changes per hour is that prescribed rates, such as the CDC’s recommendation of 5 ACH, is often excessive in large and low density spaces. Conversely, it is likely to be entirely insufficient in tiny spaces like a car. Due to the small volume, a much higher airflow rate, possibly above 50 ACH, might be required. This is possible with the use of windows or climate controls, but a tiny portable air cleaner that might provide a few ACH is not helpful in this context.
Even though the car is a small space, I would not use portable air cleaners and would instead focus on windows or climate controls to ensure clean air.
Summary
- Keep windows open when possible
- Use the climate control system when windows are closed
- Run fan as high as possible
- Turn off recirculation mode unless outdoor air pollution is a problem
- Upgrading cabin filter to a HEPA filter is a good idea, especially if a lot of time is spent in the car
- Flush the car when first entering
- Portable air cleaners are not effective
