A team of researchers has found that even a slight increase in ambient carbon monoxide levels from automobiles and other sources are associated with increased mortality.

The study, published in The Lancet Planetary Health, also found that even short-term exposure to ambient carbon monoxide (CO) — at levels below the current air quality guidelines and considered safe — had an association with increased mortality.

These findings have significant public health implications, – said researcher Kai Chen, Assistant Professor from Yale University in the US.

“Millions and millions of people live in environments with elevated CO levels and in environments where the CO levels are within the current guidelines considered ‘safe range,” Chen added.

For the study, the research team collected data from 337 cities across 18 countries and analyzed data, including a total of 40 million deaths from 1979 to 2016, and ran it through a statistical model.

Overall, a one milligram per cubic metre increase in the average CO concentration of the previous day was associated with a 0.91 per cent increase in daily total mortality, the study found.

The team also discovered that the exposure-response curve was steeper at daily CO levels lower than one milligram per cubic meter, indicating a greater risk of mortality per increment in CO exposure, and this persisted at daily concentrations as low as 0.6 mg/mA or less.

The findings reveal that there is no evidence for a threshold value below which exposure to ambient CO can be considered “safe.”

The US National Ambient Air Quality Standard for ambient CO (approximately 7 milligram per cubic metre for the daily average) was established in 1971 and has not been revisited for the past five decades.

The same air quality guideline for CO has been applied in other regions such as Europe, whereas a lower value of 4 milligrams per cubic meter was established as China’s air quality standard.

The study’s findings strongly suggest the need to revisit global and national air quality guidelines for CO and, in addition to single-pollutant standards, policies should also be expanded to address traffic-related air pollution mixtures.