What are the three most common gases tested for before entering a confined space?

The three most common gases tested for before entering a confined space are oxygen, methane, and hydrogen sulfide.

Oxygen is essential for human respiration, and a deficiency can lead to suffocation. It is crucial to ensure that there are adequate levels of oxygen before entering. The presence of methane is significant because it is a flammable gas that can create explosive atmospheres in confined spaces. Hydrogen sulfide is particularly dangerous as it is a toxic gas that can cause immediate respiratory distress or even death at high concentrations. Testing for these gases is a standard safety practice to prevent life-threatening situations in confined environments.

The other choices include gases that might not be as prevalent or relevant for confined space entry assessments, such as ammonia, which is less commonly monitored in non-agricultural workspaces, and carbon dioxide, which is typically a byproduct of respiration but isn't tested as a primary gas in confined spaces compared to those in the correct answer.