*2.3.1 Analytical method*

The reference method for the measurement of carbon monoxide concentration is based on the absorption of infrared radiation (IR) by the gas in a non-dispersive photometer [15]. CO absorbs IR radiation maximally at a wavelength of 4.7 micrometers (μm), which is in a spectral range where few other atmospheric species absorb to interfere with accurate quantification.


**Table 1.**

*WHO standard concentration of particulate matter at different stages of measurement.*

Carbon dioxide and water vapor can make major interference with the measurement of carbon monoxide. Removal of water vapor from the sample air is necessary to avoid positive interferences in the determination of CO concentration and is achieved by a permeation tube or drier that selectively removes water vapor from the sample gas without removing CO.

The concentration of CO can be determined using the Beer–Lambert law which relates the concentration of an absorbing species to the degree of light attenuation. The analytical method is suitable for stable installations at fixed-site monitoring stations.

Currently, potable carbon monoxide individual-level exposure measurements become familiar. Measurements of personal exposures are based on the electrochemical reactions between carbon monoxide and deionized water, which are detected by specially designed sensors.

The following guideline values (ppm values rounded) and periods of timeweighted average exposures have been determined in such a way that the COHb level of 2.5% is not exceeded, even when a normal subject engages in light or moderate exercise:

