The United States Congress, in the year 1960, voted to draft an Act that later evolved into the Clean Air Act in 1970 (US Environmental Protection Agency, 2012), owing to the escalation of dense smog in the country’s metropolitan cities. Thus, in the same year, under section 109 of the Clean Air Act, authorities were compelled to revise emission standards for on-road and off-road motor vehicles.
The framework and protocol circumscribing carbon monoxide emissions were introduced in 1971, with the collaborative support from the United States National Ambient Air Quality Standard (NAAQS). Owing to the metamorphosis of the carbon monoxide pollutant, the State decided to introduce a phrase called ‘Time-weight average exposure,’ under which there was a limiting value of 9 ppm for eight hours and 35 ppm for an hour outdoors, calculated cumulatively for a year.
The primary motivation for establishing the standard was to enhance public health and provide a measure to protect public welfare. The content and objectives were successively reviewed in 1985, and the US Environmental Protection Agency was yielded to maintain the primary standard without any revision. However, the secondary standard was revoked due to lack of evidence to demonstrate a direct effect of ambient CO levels on public welfare.
However, the primary criteria for the emission levels has never undergone any modifications since then, with the benchmarked points in function even today. NAAQS was developed into two stages, where the first stage was to assess the figures via statistics and probability. In contrast, the second phase consisted of various primary and secondary surveys to bolster the theoretical numbers. A pivotal basis for the study was based on the correlation between the post-exposure rise in the carboxyhemoglobin and the post-exospores time reduction of coronary artery diseases that were sensitive towards the positioning of carbon monoxide.
The Environmental Protection Agency is working closely with the State, local and tribal agencies to amend the standards set in 1971. Respective organisations are overviewing various aspects of pollution levels, including air quality index, and are using algorithms to predict future trends. In this context, the World Health Organisation has also framed guidelines for time-weighted carbon monoxide exposure on order to help prevent various physiological complexities that originate from CO emission.
The WHO recommends limiting exposure to 9 ppm CO for eight hours, 26 ppm CO for one hour, 52 ppm CO for 30 minutes, and 87 ppm CO for 15 minutes, assuming that the carboxyhemoglobin level has an elevated bracket of 2.5% when a healthy human adult has moderate exposure. Along with this assertion, the US government has laid down specific permissible standards for the indoor exposure of the carbon monoxide.
One of the examples may be cited from the Occupational Safety and Health Administration (OSHA), which recommends the carbon dioxide limiting exposure to be 50 ppm for 8 hours with a threshold concentration of 200 ppm. These figures were adopted from Underwriters Laboratories, which had initiated the process of establishing an ‘indoor carbon monoxide level’ cutoff value in 1992. The data was based on the assumption that beyond the 10% exposure of carbon dioxide in a closed environment, a patient with cardiovascular disease experiences a dramatic rise in various supplementary risks.
With the advent of climate change, the US Environmental Protection Agency is once again planning to amend the cutoff values for the carbon monoxide emission.