Peak Levels of Carbon Monoxide

Map of monitoring stations

Maximum levels of a contaminant are important because of potential health or environmental effects of high concentrations. Scientists often find the "99th percentile" concentration more useful in tracking trends than the single, maximum value. The annual 99th percentile concentration is the concentration that is higher than 99% of the concentrations recorded throughout the year. In other words, only 1% of the ambient concentrations observed during the year fall above the 99th percentile concentration.

Since the early 1990's, the annual 99th percentile carbon monoxide concentration has declined significantly. The 99th percentiles have decreased for all stations by 36 to 81% from 1990 to 2006.