A plumber notices a rotten-egg smell while opening a sewer cleanout, then the smell fades. What toxic gas is this most likely, and why is the fading smell dangerous?

Giải thích

Sewer gas commonly contains hydrogen sulfide, which smells like rotten eggs at low levels but rapidly paralyzes the sense of smell, so a fading odor can signal rising, more dangerous concentrations. Cal/OSHA sets a low permissible exposure limit for H2S because it can cause collapse and death within minutes at high ppm. Never rely on smell; use a calibrated gas meter and ventilation.