Yes, sewer gas can be deadly if inhaled in large concentrations, even while you sleep. Sewer gas primarily consists of toxic methane and hydrogen sulfide, both of which can cause asphyxiation or poisoning if levels become excessive. While rare, poorly ventilated spaces or malfunctioning sewer systems can lead to dangerous buildups of these gases, posing a significant health risk. Symptoms of exposure include dizziness, headaches, and nausea, progressing to unconsciousness or death. It's crucial to maintain proper plumbing and ventilation systems to prevent the potentially lethal effects of sewer gas, especially during vulnerable sleeping hours.