Role of time as a factor in the toxicity of chemical compounds in intermittent and continuous exposures. part II. effects of intermittent exposure

Because of fluctuations in levels of industrial air pollution linked to weather and other factors, a joint U.S.‐Soviet research team studied differences in the effect of time in exposures of air‐breathing animals to controlled varied concentrations of air pollutants. An experimental model environment was used to investigate the effects of nitrogen dixoide (NO₂), sulfur dioxide (SO₂), and benzene (C₆H₆) in the air environment of laboratory animals, with both continuous and intermittent exposures. For intermittent exposures the investigators used amounts of toxic gases whose cumulative total equaled the total in the continuous exposures, and intercalated the intermittent exposures with periods of nonpolluted air. To equal the cumulative total concentration of the single continuous exposures over an equal time period, the concentrations of toxic gases were necessarily greater in the intermittent exposures.

Part I of this work deals with the effects of continuous exposure to C₆ H₆ on the central nervous system and blood. Toxic effects of continuous exposure to various concentrations of C₆H₆ were found involving the central nervous system and blood. The time‐effect relationships could be approximated on a log‐log scale by straight lines. Similar results were obtained with SO₂.

Using different parameters to measure toxicity, similar results were obtained with NO₂ —namely, concentration‐time relationships that when plotted on a log‐log scale gave straight lines.

Part II concerns the toxicities of Intermittent exposures to C ₆ H ₆ and NO₂; three parameters were used. The toxic effects of NO₂ with intermittent exposure were not reduced as was the case with C₆H₆. Data are presented to show that intermittent exposure to NO₂ may be more toxic than continuous exposure when short‐term peaks of NO₂ concentrations are involved.