Hydrogen sulfide (H₂S) can theoretically cause rapid unconsciousness and death at high enough concentrations—yes, around 1000ppm or more—but the reality is that it's extremely volatile, difficult to control, and dangerous in practice. The reason it's not used more often is because producing a consistently lethal concentration is harder than it looks on paper, especially without exposing yourself prematurely. The gas builds up fast but it's also incredibly toxic and causes immediate respiratory distress, eye and lung irritation, nausea, dizziness, and a burning sensation especially at lower, sublethal concentrations. People report panic, vomiting, and extreme discomfort when things go wrong, which they often do. It's also highly flammable and leaves behind a horrific stench—think rotting eggs times a hundred—which can linger and alert others quickly. Unlike CO, which is odourless, H₂S gives very little margin for error and very little grace if something goes wrong. So while the PPH includes it as a method, it's really not peaceful for most people in practice, which is why you don't see it discussed as much or recommended by those with firsthand experience.