What characteristic is NOT true for methane?

Methane is indeed an odorless gas, which means that it does not have a smell that can be detected by humans. This characteristic is important for safety measures, as its flammability and potential to be explosive in certain concentrations in the air become critical considerations. Methane is also highly flammable, capable of igniting in the right conditions, which underscores the importance of proper storage and handling.

While methane can be dangerous because it can form explosive mixtures with air, it is not classified as toxic at normal ambient concentrations, making "toxic at all concentrations" the characteristic that does not accurately describe methane. In fact, methane is considered to have a very low toxicity to humans and is not harmful at typical exposure levels, though it can displace oxygen in enclosed spaces, leading to suffocation hazards if not ventilated properly. This understanding reinforces safety protocols in environments where methane might be present.