Provisional Advisory Levels (PALs) for hydrogen sulfide (H2S)

Application of Provisional Advisory Levels (PALs) protocols was performed for hydrogen sulfide (H(2)S) as experimental data permitted. Three levels (PAL 1, PAL 2, and PAL 3), distinguished by severity of toxic effects, are developed for 24-hour, 30-day, 90-day, and 2-year durations of potential drinking water and inhalation exposures for the general public. For background on the PAL program and a description of the methodology used in deriving PALs, the reader is referred to accompanying papers in this Supplement. The database includes human experimental studies, worker exposure evaluations, as well as case studies on acute and repeated exposure. The database of animal studies is substantial, covering multiple species and addressing acute, repeated, and subchronic exposure scenarios. PAL estimates were approved by the Expert Consultation Panel for Provisional Advisory Levels in November 2006. No reliable data were found on oral exposure, making it impractical to estimate PALs for drinking water. Because H(2)S exists as a gas, partitioning to air is likely to occur with an environmental release. H(2)S inhalation PAL values for 24-hour exposure are PAL 1 = 1.2 ppm; PAL 2 = 7.0 ppm; and PAL 3 = 27 ppm; the 30-day and 90-day inhalation exposure values are PAL 1 = 0.85 ppm and PAL 2 = 3.0 ppm. PAL 3 values for 30-day and 90-day exposures are not recommended due to insufficient data. Long-term data were insufficient to estimate 2-year inhalation PALs.