Improving indoor air quality is becoming an essential goal for many employers, especially during the COVID-19 pandemic. Among the list of indoor air pollutants we usually think of, like formaldehyde and particulate matter, one air pollutant is starting to become more of a concern indoors: ozone.
While ozone is usually considered a major outdoor air pollutant, the rise of certain air purification technologies designed to fight COVID-19 may also produce ozone, whether intentionally or unintentionally. This makes monitoring ozone and identifying which indoor ozone levels are safe a paramount concern.
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We’d like to take this article to discuss which indoor ozone levels are safe during building occupancy. Because ozone is usually not seen as an indoor pollutant, we will have to rely on the regulations and categorizations of ambient ozone to determine safe indoor ozone levels.
Regulatory Ozone Limits
Occupational Permissible Exposure Limits
OSHA and NIOSH have both published exposure limits for ozone, but most regulations relating to occupational exposure are designed for industrial workers.
Current OSHA regulations set an eight-hour time-weighted average permissible exposure limit (PEL-TWA) of 0.1 ppm (0.2 mg/m³). NIOSH sets a ten-hour recommended exposure limit ceiling (REL-C) at the same value, 0.1 ppm (0.2 mg/m³). These values can serve as a reference point; for occupied workspaces, indoor ozone levels should definitely be kept below 0.1 ppm.
World Health Organization Guidelines
The World Health Organization (WHO) sets an eight-hour limit at 100 μg/m³ (0.1 mg/m³), which is stricter than OSHA’s and NIOSH’s exposure limits. Ozone was going to be added to the WHO’s list of indoor air quality guidelines, but was ultimately not included in 2010 because of a lack of sufficient evidence for inclusion.
The EPA’s AQI Breakpoints
Another way we can look at ozone is through the EPA’s AQI breakpoints. While these breakpoints are used for ambient ozone, we can use them to extrapolate safe indoor ozone levels as well.
The EPA breaks down ozone exposure into two time intervals: one hour exposure and eight hour exposure. The tables below summarize various ozone levels and their safety rating for eight-hour exposure.
|AQI Category||AQI Value||Ozone Level (ppm)|
|Unhealthy for sensitive individuals||101-150||0.071-0.085|
The EPA’s AQI breakpoints provide some more information than OSHA’s, NIOSH’s, and WHO’s exposure limits. The breakpoint for “good” air quality is 0.054, which converts to about 0.106 mg/m³, about the same as the WHO’s recommended limit of 0.1 mg/m³.
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We can learn two things from our discussion of safe indoor ozone levels.
- We currently don’t have a definitive way to frame indoor ozone. In order to identify safe indoor ozone levels, we had to use industrial exposure limits and ambient ozone breakpoints. These figures are not ideal for an indoor setting, like an office.
- If we abide do abide by outdoor limits, then indoor ozone levels should be kept below 0.1 mg/m³, as is in line with the WHO's guidelines and the EPA's breakpoints.
Ozone doesn’t just originate from the outdoors; some electronics and air purification technologies can produce ozone. To learn more, check out our informative article: 3 Common Sources of Indoor Ozone