A recent meeting of the Carbon Monoxide (CO) Working Group convened to discuss various aspects of establishing air quality standards for this critical pollutant. The discussion spanned from the applicability of space-dependent limits to the technicalities of sensor technology and the units used for measurement. This article summarizes the key deliberations and the apparent direction the working group is taking.

Space Adaptability and Time Averaging

The initial point of discussion revolved around space adaptability, the concept of having different CO limits for different locations. Survey feedback revealed divided opinions within the community, with some suggesting changes and others proposing target limits. A significant concern was raised that averaging concentrations over longer periods could mask periods of high exposure, such as during school days in classrooms.

Furthermore, the group addressed the fundamental issue of interpreting existing regulatory standards, which often specify time-weighted averages. A key point of contention was the common practice among consumer product manufacturers of using these average limits as immediate ceilings to trigger alerts. While this approach is considered more protective, it deviates from the intended use of the standards. The working group explored the implications of explicitly adopting this more protective interpretation.

Sensor Capabilities and Concerns

The technical aspects of CO sensors were also thoroughly discussed. Concerns were raised about instruments misunderstanding average limits and using them as simple notification thresholds. A member highlighted the challenge of cross-sensitivity with hydrogen, which can affect CO readings, especially indoors.

The working group emphasized the importance of distinguishing between air quality monitoring and safety devices for CO. Safety-rated CO alarms adhere to specific regulations regarding alarm thresholds and response times, often at much higher levels than what would be considered safe for long-term air quality. The potential for a “false sense of protection” when using air quality monitors for safety purposes was a significant concern. It was clarified that consumer-grade air quality instruments generally allow users to set their own thresholds and are not intended as safety devices.

The Debate on Units: PPM vs. PPB vs. Milligrams per Cubic Meter

A significant portion of the discussion focused on the most appropriate unit for measuring and communicating CO levels. While current practices often use parts per million (ppm) or milligrams per cubic meter, a strong sentiment emerged in favor of adopting parts per billion (ppb). The rationale for this shift included:

More accurate representation of low levels: Using ppm can give a false impression of zero at very low, yet potentially impactful, concentrations. Parts per billion offers finer resolution for these levels.

Consistency with other pollutants: Measuring CO in ppb would align it with the measurement units used for other gases like nitrogen oxide and ozone, making comparisons easier for the public. It was suggested this could highlight that CO levels often exceed those of other monitored pollutants.

Critique of milligrams per cubic meter: This unit requires conversions based on temperature and atmospheric pressure, which consumer devices typically don’t measure or don’t apply, making the reported values potentially misleading. The historical basis for using mass-based units for gases was acknowledged, but the practical advantages of ppm/ppb for current instrumentation were emphasized.

The working group seemed to lean towards recommending ppb as the primary unit, potentially with an option for instruments to display in other units, albeit with caution regarding inaccurate conversions.

Reviewing Existing Standards and Setting Protective Limits

The group reviewed various existing CO standards and guidelines from different countries and organizations, noting the wide range of recommended limits and averaging times. There was a clear interest in adopting the most stringent and protective values for the GO AQS. The German standard for unlimited safe exposure at a very low level was specifically highlighted. The inadequacy of older standards set when ambient CO levels were significantly higher was also discussed in light of recent epidemiological studies showing health impacts at very low ppm levels.

Health Effects and Exposure Levels

The discussion addressed both the acute risks of high CO concentrations and the potential for chronic health effects from low-level exposure. Recent research highlighting increased mortality even with small increases in ambient CO levels (around 1 ppm) generated considerable discussion. The importance of clearly differentiating between the risks of low-level, long-term exposure and the immediate danger of high concentrations was emphasized. While disability outcomes were acknowledged as important, a suggestion was made to ensure mortality data is also prominently considered. The need for more research on the health effects of chronic low-level CO exposure was recognized.

Scope and Instrument Display Ranges

The working group clarified that the GO AQS standards are intended for consumer-controlled, non-occupational settings, explicitly excluding manufacturing facilities, warehouses, which fall under occupational safety regulations.

A key discussion point concerned the display range of consumer CO monitors. In the US, CO alarms often have a “blackout” range below 30 ppm. The working group discussed the need for air quality monitors to display CO levels below this threshold, potentially starting as low as 0.1 ppm (100 ppb), to provide consumers with more comprehensive information. It was suggested that a minimum display range of 30 ppm could address the US alarm blackout issue, while allowing manufacturers to go higher (e.g., 300 or 3,000 ppm) to capture more extreme events.

Conclusion and Next Steps

The Carbon Monoxide Working Group engaged in a comprehensive discussion covering various critical aspects of setting air quality standards for CO. Key trends and potential conclusions emerging from the meeting include:

A move towards interpreting regulatory average limits as immediate ceilings for consumer devices to enhance protection.

A strong consideration of adopting parts per billion (ppb) as the preferred unit for measuring and reporting CO levels.

An emphasis on requiring consumer monitors to display lower levels of CO, potentially down to 100 ppb, to provide more complete exposure information.

A clear focus on consumer-controlled, non-occupational environments as the scope of GO AQS recommendations.

A commitment to reviewing and potentially adopting the most stringent available CO standards.

The next steps involve summarizing the discussion, potentially conducting further votes on key issues like the limits for adoption, and outlining the future work of the advisory committee. The insights shared during this meeting will be crucial in shaping the GO AQS standards for carbon monoxide, aiming for greater public awareness and protection from this pervasive pollutant.

• Recording: Carbon Monoxide WG