WHO has concluded that despite the large number of acute human exposure studies

there is no evidence for a clearly defined concentration/response relationship for NO2

exposure. For acute exposures, only very high concentrations (> 1880 Tg/m3) effect

healthy people. Asthmatics and patients with chronic obstructive pulmonary diseases

are clearly more susceptible to acute changes in lung functions, increased airway

responsiveness, and symptoms. Based on small changes in lung functions and changes

in airway responsiveness in several studies, a range of 375 to 565 Tg/m3 over a 1- to

2-hour exposure is a clear lowest-observed-adverse-effect level (LOAEL). A 50 %

margin of uncertainty is included in the WHO recommendations because of the

reported statistically significant increase in response to increased airway

responsiveness with exposure to 190 Tg/m3 and a meta analysis suggesting changes in

airway responsiveness. Based on human clinical data, a 1-hour guideline of 200 Tg/m3

is recommended as a maximum value. At double this recommended guideline, there is

evidence to suggest possible small effects in pulmonary function of asthmatics.

Should an asthmatic be exposed either simultaneously or sequentially to NO2 and an

aero-allergen, the risk of an exacerbated response to the allergen is increased. At 50 %

of the suggested guideline (i.e. 100 Tg/m3) no acute exposure studies have been

undertaken, so effects are unknown.

 

 

 

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