Since much of the sulfur dioxide (SO₂) that is produced from fossil fuel combustion is oxidized to sulfuric acid, (H₂SO₄), there is concern about the effects of inhaled H₂SO₄ on human health. Adolescent asthmatic subjects, unlike healthy or even adult asthmatic subjects, have shown decreased pulmonary function after inhalation of 100 μg/m³ H₂SO₄. This paper describes a recent study exploring the effect of inhalation of a near ambient concentration of H₂SO₄ in allergic adolescent subjects with asthma. Subjects were exposed via a rubber mouthpiece for 30 min during rest and 10 min during moderate exercise to filtered air, 68 μg/ m³ H₂SO⁴, or 0.1 ppm SO₂ alone or in combination. Pulmonary functions (forced expiratory volume in one second FEV₁, total respiratory resistance R_T and maximal flow) were measured before and after exposure. Exposure to 68 μg/m³ alone caused a significant 6% decrease in FEV₁ (p = 0.012), whereas exposure to air or SO₂ alone did not. Exposure to SO₂ combined with the H₂SO₄ aerosol caused a larger increase in R_T (15%) than the single exposures. These results are contrasted with similar data for adult asthmatic subjects, which failed to find pulmonary function changes.