Airway Effects of Low Concentrations of Sulfur Dioxide: Dose-Response Characteristics

ABSTRACT

The acute respiratory effects of exposure to low-level, short-term sulfur dioxide (SO₂) were studied in ten asthmatic and ten healthy subjects. Subjects were exposed in an environmental chamber in a double-blind, random sequence to SO₂) levels of 0.00, 0.25, 0.50, 0.75, and 1.00 ppm for 40 min. During the first 10 min subjects exercised on a cydoergometer at a level of 450 kpm/min. On separate days, subjects were exposed to 0 and 1.0 ppm SO₂ in the absence of exercise. In exercising asthmatic subjects, breathing 1 ppm SO₂ resulted in significant changes from baseline in airway resistance (Raw), forced expiratory volume in 1 sec (FEV₁), maximal expiratory flow at 60% of the vital capacity (VC) below total lung capacity on the partial flow volume curve [MEF40%(P)] as well as reductions in flows at 50% of VC ( _max50%). None of these parameters showed significant changes for exposures to 0.5 ppm or less, with the exception of small decreases (i.e., < 0.2 L/sec) in _max50% at 0.25 and 0.5 ppm. Ten minutes after the end of exercise, there were no statistically significant differences from baseline, even though SO₂ was still present in the chamber atmosphere. For asthmatic subjects, the average changes in Raw, FEV₁₀, MEF40%(P), and V_max50% increased as SO₂ levels increased, suggestive of a dose-response relationship with a consistent effect first seen at 0.75 ppm. In individual exercising asthmatics, responses may occur at levels of S0₂ below 0.75 ppm. No changes were seen in healthy individuals on any day, or in asthmatic subjects at rest. These results indicate that asthmatic individuals engaged in moderate activity have transient bronchoconstriction following exercise in the presence of S0₂, with consistent changes first being noted for the group when ambient levels reach 0.75 ppm.