At what AQI should you wear a mask? A Reddit cyclist recently put this question to the r/AirQuality community: at what AQI do you actually put on a mask outdoors? They walk and bike commute daily, and mask up once the index passes about 62 — even though nobody around them masks until the air is officially “unhealthy.”
Many people in the discussion suggested a threshold between 40 and 70. Some described headaches, throat irritation or a vague malaise while the index still read “moderate.”
That is not a scientific sample, and individual symptoms are not a reliable way to determine whether air pollution is harmful. But the discussion raises a useful question: should the same AQI threshold apply to someone sitting indoors, a cyclist riding hard for an hour, and a child walking to school?
The answer is not necessarily.
The US AQI categories are designed to communicate population-level air-quality risk. Your practical threshold can be lower when you exercise outdoors, spend a long time near traffic, or are more sensitive to air pollution. This guide explains what the AQI measures, how to read it, and gives you a practical threshold table based on exposure and activity rather than the headline category alone.
Note for readers in Singapore and Southeast Asia: your local authorities use the PSI rather than the US AQI. The two systems are not directly interchangeable. Singapore also publishes 1-hour PM2.5 readings, which are particularly useful when deciding what to do outdoors right now. We explain the difference below.
What is the Air Quality Index (AQI)?
The Air Quality Index (AQI) is a way of turning measurements of several air pollutants into a number that is easier to understand. The US AQI covers five major pollutants: ground-level ozone, particle pollution including PM2.5 and PM10, carbon monoxide, sulfur dioxide and nitrogen dioxide. The US scale runs from 0 to 500, with higher numbers indicating greater health concern.
The official US categories are:
- 0–50: Good
- 51–100: Moderate
- 101–150: Unhealthy for Sensitive Groups
- 151–200: Unhealthy
- 201–300: Very Unhealthy
- 301+: Hazardous
See the US EPA AirNow AQI guide for the official definitions.
A key point many people miss is that the headline AQI reflects the single pollutant with the highest AQI sub-index at that time. That matters because a high AQI caused by particulate pollution is a different problem from a high AQI caused by ozone.
A practical AQI guide for everyday outdoor activity. The appropriate response depends on the pollutant, exposure time and activity level.AQI Thresholds: When Should You Put a Mask On?
There is no single AQI number that makes a mask necessary for every person in every situation.
Exercise changes the calculation because you breathe more rapidly and deeply. A cyclist or runner can therefore inhale substantially more air during an hour outdoors than someone sitting quietly. Children and people who are more sensitive to air pollution may also reasonably choose a lower threshold.
The table below is therefore a practical exposure guide, not an official medical threshold. It is intended to help you decide when a particulate respirator may be useful.
The key reference point is around AQI 63. Under the US EPA PM2.5 AQI breakpoints, this corresponds to roughly 15 µg/m³ of PM2.5, close to the WHO 24-hour PM2.5 guideline of 15 µg/m³. The WHO air-quality guidelines provide the scientific health-protection reference; they are not themselves an AQI threshold for mask use.
| AQI (US) | PM2.5 µg/m³ |
Category | Most people | Cycling, running, children & sensitive airways |
|---|---|---|---|---|
| 0–50 | 0–9.0 | Good | No mask normally needed | No mask normally needed |
| 51–62 | 9.1–15 | Moderate | Usually no mask needed | Consider a mask for longer or harder outdoor activity |
| 63–100 | 15.1–35.4 | Moderate above the WHO 24-hour guideline |
Consider a mask if outdoors for an hour or more | Wear a mask when exposure is prolonged or strenuous |
| 101–150 | 35.5–55.4 | Unhealthy for Sensitive Groups | Consider wearing a mask | Mask strongly recommended for prolonged outdoor activity |
| 151–200 | 55.5–125.4 | Unhealthy | Mask recommended; reduce exposure | Mask essential; reduce exercise intensity |
| 201–300 | 125.5–225.4 | Very Unhealthy | Mask essential; limit time outdoors | Avoid strenuous outdoor exercise |
| 301–500 | 225.5+ | Hazardous | Stay indoors; mask if exposure is unavoidable | Stay indoors |
PM2.5 conversions follow the US EPA AQI breakpoints. The practical advice in the two right-hand columns is Totobobo’s exposure-oriented guidance, not an official EPA or WHO mask recommendation. An hour of hard cycling at AQI 80 also represents a much greater inhaled dose than an hour of sitting quietly at the same AQI.
First, check what is actually driving the number
A headline AQI reflects the pollutant with the highest sub-index at that moment. That pollutant is not always particulate matter.
On hot, sunny days, the driver may be ozone, which is a gas rather than a particle.
A particulate filter does not remove ozone. N95, KN95 and FFP3 particulate respirators are not designed to remove ozone, and neither are Totobobo’s particulate filters.
If ozone is driving your local index, reducing exposure is the better response: ride earlier in the day, choose a lower-exposure route, reduce strenuous activity or move the workout indoors.
When PM2.5 or PM10 is the main pollutant — for example during wildfire smoke, haze, traffic pollution or dust events — a properly fitted particulate respirator can reduce the particles you inhale.
You can usually see the pollutant breakdown in your local air-quality app or monitoring service. Check what is driving the number before assuming a mask is the answer.
Why mask fit matters as much as filter rating
The second catch with any threshold is that a particulate respirator only helps if the air you breathe actually passes through the filter.
A loose mask can allow polluted air to enter around the edges. That is why facial fit matters alongside filter efficiency.
Our guide explains how to check your mask fit and seal, including Totobobo’s visual seal-check method.
For the evidence behind filtration and fit, see Why respirator fit matters: independent research behind Totobobo.
On feeling like the only one wearing a mask
One practical barrier to outdoor masking is social rather than technical: you may feel conspicuous when nobody else is wearing one.
But the number of people wearing masks around you is not a measure of air quality. If pollution exposure is high enough for your circumstances, being the only person taking precautions does not make the precaution unnecessary.
There is also a design question. The Totobobo shell is transparent and sits close to the face rather than projecting outward like a conventional cartridge respirator. Why did we make it transparent?
Who Is Most at Risk from Poor Air Quality?
Poor air quality can affect everyone, but the impact depends on the pollutant, concentration, duration of exposure, activity level and individual health.
Groups that may need to take greater care include:
- Children — their lungs are still developing and they may have greater exposure relative to their body size. During polluted school commutes or prolonged outdoor activity, parents may reasonably choose a lower threshold. See our guide to the transparent children’s anti-pollution mask.
- Older adults — older people can be more susceptible to the respiratory and cardiovascular effects of air pollution.
- People with asthma, COPD, heart disease or other relevant conditions — air pollution can aggravate existing respiratory or cardiovascular problems. If you have a medical condition, follow advice from your healthcare professional rather than relying on a generic AQI threshold.
- People who exercise or work outdoors — cyclists, runners, construction workers and athletes breathe more deeply and therefore can inhale a larger dose during the same period of exposure.
For cyclists, the combination of exercise and traffic exposure deserves particular attention. Our detailed guide explains how cycling in polluted air changes your exposure and when a mask may help.
How to Check Your Local Air Quality
Use an official local monitoring service where one is available. For international readers, these services can also be useful:
- United States: AirNow / US EPA AQI
- Global: World Air Quality Index provides a global map and local monitoring data.
- Global: IQAir / AirVisual provides air-quality readings, alerts and historical information.
- Singapore: NEA’s air-pollution guidance explains the 24-hour PSI and 1-hour PM2.5 readings. You can also check the latest readings through Singapore’s haze portal and the myENV app.
On any of these services, look beyond the headline number and check the pollutant breakdown. You want to know whether PM2.5, PM10 or ozone is driving the reading because a particulate mask is useful for particles, not ozone.
For Singapore, there is another important distinction: NEA says the 1-hour PM2.5 reading is intended to help with immediate outdoor decisions, while the 24-hour PSI and forecast are more useful for planning activities over the coming day.
If you cycle in Singapore, our latest guide explains how to use these readings during haze: Singapore Haze: What Cyclists Need to Know About PM2.5.
AQI vs PSI: What If You Live in Singapore?
Do not assume that a Singapore PSI number is equivalent to the same number on the US AQI scale.
The two systems use different calculations, pollutants, averaging periods and breakpoints. Singapore’s 24-hour PSI is designed to communicate longer-term daily air-quality conditions, while NEA’s 1-hour PM2.5 concentration provides a more immediate indication of current fine-particle pollution.
This distinction becomes particularly useful during haze. A cyclist deciding whether to go out for a ride now should look at the current local PM2.5 conditions as well as the broader PSI forecast, rather than relying on one number alone.
See NEA’s explanation of PSI and 1-hour PM2.5 for the official Singapore guidance.
Frequently Asked Questions
What AQI is dangerous for cycling?
There is no single AQI value that is “dangerous” for every cyclist. However, AQI 63 is a useful practical point to start thinking seriously about particulate exposure, especially for prolonged or strenuous riding, because it corresponds to roughly 15 µg/m³ PM2.5 under the US AQI system.
Above 101, the US AQI enters the “Unhealthy for Sensitive Groups” category. For strenuous outdoor exercise, consider reducing exposure or using a properly fitted particulate respirator. Above 151, reducing exercise intensity becomes increasingly important, and above 201 strenuous outdoor exercise is better avoided.
If you ride regularly in traffic, see our detailed guide to cycling masks for pollution.
At what AQI should children wear a mask?
Children are one group for whom a lower exposure threshold can be reasonable. For a polluted school commute or prolonged outdoor activity, consider particulate protection from around AQI 63 when PM2.5 is driving the reading, and take stronger precautions as the AQI rises.
The important practical issue is fit. A mask must seal properly on a smaller face. See our guide to the Totobobo children’s mask.
Will a mask help if the AQI is high because of ozone?
Not a standard particulate mask. Ozone is a gas, while N95, KN95, FFP3 and Totobobo particulate filters are designed to capture particles rather than remove ozone.
If ozone is driving the AQI, reduce exposure instead: ride earlier in the day, avoid strenuous outdoor exercise during high-ozone periods, choose a cleaner route or move the activity indoors.
Does a “good” AQI mean the air is completely safe?
No. “Good” is a category defined by the AQI system, not a guarantee that every pollutant is absent or that every person will experience no effect.
The AQI also focuses on the pollutants included in that index. If you have asthma or another condition that makes you sensitive to air pollution, follow your healthcare professional’s advice even when the headline AQI looks good.
What if I am in Singapore and the PSI looks fine?
During haze or rapidly changing pollution conditions, also check the 1-hour PM2.5 reading for your region. NEA specifically provides this reading to help with immediate outdoor decisions.
For cyclists, see our current guide to Singapore haze and PM2.5.
Protect Your Lungs When the Numbers Climb
Knowing the AQI is only useful if you act on it appropriately.
If particulate pollution is high, a properly fitted particulate respirator can reduce the particles you inhale. But the filter rating is only part of the equation. The mask also needs to seal to your face.
Our mask fit and seal guide explains how to check the fit, while our independent research summary explains why fit and filtration need to be considered together.
- Ride through traffic? ? The cycling mask for pollution
- Protecting a child? ? The transparent children’s mask
- Wear glasses? ? See how Totobobo works with spectacles
- Want to understand the filter? ? How can a filter capture sub-micron particles?
The simple rule
Don’t look at AQI alone.
Check what pollutant is driving the number, how long you will be exposed, how hard you will be breathing, and whether your mask actually seals.
For particulate pollution, the higher the exposure and breathing rate, the more useful a properly fitted particulate respirator becomes. For ozone, reducing exposure is the better strategy.