A recent session of the GO AQS working group convened to discuss and propose limits for nitrogen dioxide (NO2) in indoor environments. The discussion involved various aspects, including averaging times, concentration limits, units of measurement, and the feasibility of monitoring.

Setting the Stage: Averaging Times and Ceiling Limits

The session began with an overview of existing nitrogen dioxide guidelines, highlighting the variability in average times (15 minutes, 1 hour, 8 hours, 24 hours, and 1 year) and the presence of ceiling limits. A key question raised was whether to link average time concentrations with different indoor environments and whether to opt for a single ceiling limit instead of multiple average durations.

The idea of a single ceiling limit was debated. Some participants noted its simplicity for developing an air quality index. However, concerns were raised that a single, stringent ceiling limit might be unachievable or trigger frequent false positives due to sensor limitations. Conversely, a less stringent ceiling might not adequately protect against both short-term spikes and long-term exposure. The potential for misleading the public with a single safe-appearing number, even if short-term spikes exceed it, was also highlighted.

Proposed Concentration Limits and Comparisons

The working group considered proposed limits based on existing standards, aiming for stringent levels. The initial suggestions were:

• WHO sets NO2 at 10 μg/m³ for 1-year mean (2021)
• Canada sets NO2 at 21 μg/m³ for 24-hour mean (2015)
• Singapore sets NO2 at 40 μg/m³ for 8-hour mean (2016)
• UK sets NO2 at 40 μg/m³ for 1-hour mean (2018)

However, it was noted that the 1-hour proposal of 40 micrograms per cubic meter, reportedly from the UK, might have been a misunderstanding and was likely intended as a long-term average. Comparisons with the World Health Organization (WHO) guidelines revealed that the WHO provides values for annual (10 µg/m³) means, but not for 8-hour. The WHO’s 24-hour guideline was noted at 25 µg/m³.

A member, proposed alternative limits based on a combination of long-term and short-term exposure considerations, as well as sensor capabilities:

• Annual mean: 10 μg/m³
• 8-hour mean: 40 μg/m³
• 1-hour mean: 200 μg/m³

This proposal acknowledged the importance of capturing both chronic and acute exposures.

Sensor Technology and Monitoring Challenges

The limitations of current low-cost nitrogen dioxide sensors were a significant point of discussion. Concerns about limited accuracy, cross-sensitivity to other oxidizing gases like ozone, and potential for false positives at very low limits were raised. It was suggested that overly stringent limits might trigger frequent false alarms due to sensor inaccuracies.

However, a counter-argument emphasized that standards should be based on health effects, and sensor technology should evolve to meet those standards. One sensor manufacturer indicated that they are developing more sensitive materials capable of measuring low ppb levels and addressing cross-sensitivity through filter packaging.

Indoor Sources of Nitrogen Dioxide

The discussion touched upon common indoor sources of nitrogen dioxide. Gas stoves were identified as a significant contributor, potentially reaching levels close to long-term limits during use. Other sources include heating appliances using natural gas, poorly ventilated gas boilers, and infiltration of outdoor pollution from traffic, especially in areas with high diesel vehicle use. Even attached garages could allow vehicle exhaust to enter residential spaces.

Units of Measurement: Micrograms per Cubic Meter vs. Parts per Million/Billion

The working group addressed the question of the most suitable unit for measuring nitrogen dioxide. While WHO uses micrograms per cubic meter, researchers and sensor manufacturers often work with parts per million (ppm) or parts per billion (ppb).

The complexity of converting between these units based on temperature and atmospheric pressure was discussed. While a standardized conversion at 25°C and 1 atmosphere is often used for simplicity, it can introduce a degree of error. It was suggested that sensors could display values in ppb/ppm for ease of communication, with the underlying data on temperature and pressure available for more accurate conversions to micrograms per cubic meter when needed for compliance or research purposes. Examples were provided to illustrate the relatively small difference in converted values within typical indoor temperature ranges.

Focus on Health Effects and Setting Limits

The importance of considering health effects, particularly the link between NO2 exposure and asthma exacerbation, especially in vulnerable populations like school children, was emphasized. This led to a reconsideration of the 1-hour mean limit. While 200 micrograms per cubic meter was a commonly found guideline, a proposal to lower it to 100 micrograms per cubic meter gained traction due to health concerns. This lower value was seen as more protective, especially against short-term spikes that could trigger asthma attacks.

Tentative Conclusions and Next Steps

While no definitive conclusions were reached on all aspects, the working group showed a tendency towards:

Adopting multiple averaging periods for nitrogen dioxide limits, including annual, 8-hour, and 1-hour means.

Seriously considering a 1-hour mean limit of 100 micrograms per cubic meter based on health considerations, despite the feasibility for sensors being potentially challenging.

Maintaining a long-term (annual) limit of 10 micrograms per cubic meter.

Further deliberation on the 8-hour and potentially a 24-hour mean around the 40 and 25 micrograms per cubic meter levels, respectively.

Exploring the possibility of using ppb/ppm for communication alongside the ability to convert to micrograms per cubic meter using temperature and pressure data.

The facilitator indicated that the feedback from the session, particularly regarding the proposed values and the idea of ceiling limits versus average times, would be taken into consideration. Further consultation with academics in the field is planned before another survey is conducted to determine the final limit recommendations. The development of an air quality index will follow the establishment of these limits.

• Recording: Nitrogen Dioxide WG – 2025 April 03