The Ozone Working Group (WG) convened on April 2nd, 2025, as part of the Global Open Air Quality Standards initiative, to delve into the specifics of incorporating ozone (O₃) into indoor air quality guidelines. Following previous discussions on particulate matter, carbon dioxide, and carbon monoxide, this session focused on the unique challenges posed by ozone, with participants raising important points regarding the establishment of target and ceiling limits, and appropriate measurement units.

The Debate on Space Adaptability and Setting Limits

A central theme of the discussion revolved around the space adaptability feature, a concept that would apodt ozone limits based on the specific environment. The initial idea aimed to accommodate varying ventilation needs and activity levels, such as in hospitals or fitness centers where occupants might inhale significantly more air. However, some participants voiced concerns about the complexity and potential for confusion that space adaptability could introduce for both the working group and companies developing air quality monitoring instruments.

The discussion leaned towards adopting a ceiling limit (maximum), mirroring the approach taken with other pollutants. One participant suggested that the decision on adaptability for ozone should align with decisions made for other pollutants. The possibility of extrapolating a ceiling limit from existing average limits in common standards was also explored.

A key argument against space adaptability was its complexity in communication and implementation. One participant suggested that a simpler, more easily understood approach should be prioritized for the Global Open Air Quality Standards. Furthermore, it was noted that the existing literature on standards does not generally support the logic of adjusting limits based on occupancy or activity levels.

Concerns Regarding Ozone Sources and Indoor Environments

The working group addressed the crucial distinction between naturally occurring ground-level ozone that infiltrates indoor spaces and ozone generated by indoor sources such as electronic devices. Concerns were raised about air purifiers marketed as “ozone-free” that may still produce ozone. It was highlighted that regulations for air purifiers aren’t always strict, and testing conditions might not reflect real-world usage.

Participants noted that ozone is an effective method for killing bacteria and viruses, leading to its use in some air purification technologies. However, the potential for these devices to generate harmful levels of ozone as a byproduct is a significant concern. Other indoor sources of ozone mentioned included UV lamps and even some cleaning products containing peroxide, which can accelerate ozone formation.

The behavior of outdoor ozone entering indoor environments was also discussed. While some research suggests it might be converted into other unknown compounds within ducting systems and other indoor surfaces, the implications of this conversion require further understanding.

Regulatory Landscape and Measurement Units

The working group acknowledged the existence of regulations restricting ozone production from indoor equipment, citing the UL standard (UL 867) in the United States, which sets a very low limit (around 5 ppb). However, it was pointed out that in some geographical areas with high outdoor ozone levels, such as Southern Europe, achieving such low indoor levels can be challenging even with closed windows. Outdoor ozone levels fluctuate throughout the day, influenced by sunlight and interactions with nitrogen dioxide and VOCs.

A significant part of the discussion focused on the appropriate units for measuring ozone concentration. While micrograms per cubic meter (µg/m³) is commonly used for various pollutants by WHO, there was a suggestion to adopt parts per billion (ppb), particularly because ozone is a gas and reference monitoring stations often use ppb. Conversion between these units can be problematic without knowing the temperature and atmospheric pressure at the time of measurement.

The scientific and epidemiological communities often use micrograms per cubic meter to correlate pollutant levels with health effects. However, for everyday measurements in buildings, ppb might be more commonly used in practice. A potential compromise discussed was to recommend the use of parts per billion while providing guidance on conversion factors considering measurement conditions for accuracy, especially when relating measurements to health effect studies.

Health Effects and Sensory Perception of Ozone

The discussion touched upon the health effects of ozone, noting that some literature suggests that higher concentrations might be needed to observe significant effects compared to other pollutants. However, ozone is a known oxidant and can irritate the airways and even the eyes at sufficient concentrations.

The possibility of including information on the sensory perception of ozone at different concentration levels was also raised. This could help individuals understand their indoor environment and potential issues with ozone-generating devices. It was also mentioned that even common household items like ceiling fans and electronic devices can produce small amounts of ozone.

Potential Conclusions and Next Steps

Based on the discussions, several potential conclusions or directions emerged:

There is a strong inclination to drop the space adaptability feature for ozone limits due to its complexity.

Adopting a ceiling limit for ozone appears to be a favored approach, potentially based on the strictest existing standards found (around 100 µg/m³ or 50 ppb).

The working group is considering recommending the use of parts per billion (ppb) for ozone measurements, aligning with common practices in scientific monitoring, while providing guidance on conversion to micrograms per cubic meter.

The importance of considering indoor sources of ozone, particularly air purifiers and cleaning products, remains a key concern.

The influence of outdoor ozone levels on indoor air quality needs to be taken into account.

Moving Forward

Following the initial discussions and data gathering on ozone limits, the next crucial step involves soliciting further input from the working group members. To this end, an additional targeted survey will be distributed shortly. This survey is specifically designed to gather expert opinions and insights necessary to establish a definitive limit for ozone within the GO AQS. The responses received will be instrumental in setting a protective and achievable standard for this pollutant.

• Recording: Ozone WG – 2025 April 02