Nitrogen dioxide: evaluation of human health risks in chamber studies

Report no. 96/59: In 1995 the World Health Organization/European region (WHO/EU) proposed a 1-hour exposure guideline of 0.11 ppm for NO₂. This guideline is based on a 50%uncertainty factor applied to a LOAEL of 0.20 to 0.30 ppm for 1 to 2-hour experimental exposures in chamber studies. The health endpoints used by WHO are: a) small reductions in lung function (>5%∗drop in FEV1 after adjustment for air exposure), and b) changes in airway responsiveness.

The 50% uncertainty factor is based on a possible (because of an "inappropriate statistical analysis") increased airway responsiveness observed in one study at 0.10 ppm (Orehek et al., 1976) and a meta-analysis suggesting increased airway responsiveness below 0.2 ppm (Folinsbee, 1992). Asthmatics and patients with COPD are said by WHO to be "clearly more susceptible" for reductions in lung function, increased airway responsiveness, and symptoms than are healthy persons. WHO indicates the LOAEL for "possible small effects in pulmonary function of asthmatics" is about 0.21 ppm, and simultaneous and/or sequential exposures to NO₂ and an aero allergen increase the risk of an exacerbated airway response.

The LOAEL for healthy persons is greater than 1.0ppm. Thus, the rationale for the 0.11 ppm NO₂ proposal is to avoid changes in airway responsiveness and avoid exacerbation of airway constriction if concomitant exposure to aero allergens and NO₂ were to occur.

∗ On page 8 of WHO (1995), the recommended guideline is based on 5% as in the referenced studies, the reduction inFEV1 is >5%.