Carbon monoxide exposures in vulnerable groups: Use of a toxicokinetic model to evaluate current indoor air guidelines and standards

Abstract

BACKGROUND: Carbon monoxide (CO) is a common indoor air pollutant. CO binds to haemoglobin (Hb) to form carboxyhemoglobin (COHb), thereby reducing blood oxygen (O2). Health Canada (HC) and World Health Organization (WHO) use 2% COHb to establish indoor air guidelines, the lowest level at which a health outcome has been found (i.e. exercise induced dyspnoea in subjects with cardiovascular disease). CO effects on other physiologic vulnerabilities are not well known, including those in persons with respiratory disease (possibly higher CO uptake due to air trapping), anaemia (higher endogenous CO production and lower Hb) and elderly (reduced capacity to respond to lower blood O2). AIM: To use a validated toxicokinetic model to predict COHb in vulnerable groups (persons with respiratory disease, anaemia, elderly) resulting from exposure scenarios corresponding to indoor air guidelines and standards. METHODS: We reviewed experimental studies investigating CO exposure and COHb in healthy and vulnerable groups. Levels were compared with simulated COHb based on exposure history and parameters specific to each group, and exposure scenarios corresponding to HC and WHO guidelines. Finally, we assessed CO levels required to reach COHb associated with known health outcomes in vulnerable groups. RESULTS: We identified 2 studies for persons with respiratory disease (COPD), 1 for anaemics, and 1 for healthy elderly. Modelled COHb for all exposure scenarios were comparable for cardiac, healthy and elderly persons. Exposures corresponding to HC and WHO guidelines led to COHb levels >2% in anaemics for all scenarios and in COPD subjects for longer exposures (i.e. 24 hrs). CONCLUSION: Our findings suggest that existing guidelines may not be protective of all vulnerable groups, particularly chronically exposed COPD patients and anaemics. More research is needed to better understand CO effects in vulnerable groups. .