Which gas is commonly referred to as the "Silent Killer"?

Carbon monoxide is commonly referred to as the "Silent Killer" due to its colorless, odorless, and tasteless properties, making it particularly dangerous. This gas is produced during the incomplete combustion of fossil fuels and can accumulate in enclosed spaces, leading to poisoning without any warning signs. Symptoms of carbon monoxide poisoning can mimic other illnesses, making it difficult for victims to recognize the danger until it's too late.

In contrast, other gases listed do not share these same characteristics. Oxygen is essential for life, carbon dioxide is naturally occurring and can be sensed by the body through increased respiration when levels are too high, and nitrogen is abundant in the atmosphere but does not pose a direct health risk in typical concentrations. Thus, the unique properties and effects of carbon monoxide make it a significant concern for safety, especially in indoor environments or spaces with poor ventilation.