Review of WHO regional office for Europe proposed short-term SO2 air quality guideline

Report no. 96/58: The World Health Organization (WHO) regional office for Europe recommends an SO₂ guideline of 500 μg/m3 (0.175 ppm) for a 10-minute sampling time, an uncertainty factor of 2, and a threshold of about 1000 μg/m3 (0.35 ppm).

Based on SO₂ chamber studies described herein the following conclusions are summarized:

- Population at risk the WHO-EU recommended guideline is set to protect the most susceptible individuals among the most susceptible population, namely exercising asthmatics not receiving medication.

- 10-minute sampling time. The data suggest an exercising asthmatic's response to SO₂ begins within minutes, reaches a maximum in about 10 minutes, and does not increase with longer exposures (and may actually decrease with longer or repeated exposures). Further, broncho constriction may be reduced at rest compared with that experienced while exercising even if exposure continues.

- Healthy subjects, COPD patients, and resting asthmatics are not at risk of an adverse response to SO2 exposure at concentrations as high as 1 ppm SO₂.

- The interpretation of the exposure-response relationships of exercising asthmatics, depends on the definition of an adverse effect. The WHO-EU definition is the most conservative one in the literature.

- The basis for inclusion of an uncertainty factor of two is unclear. The chamber studies include sensitive subjects, representative of the asthmatic population.

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