Question: Why can we not breath on mars?

Zsolt Keszthelyi answered on 13 Nov 2020:

The atmosphere on Mars consists of different elements (mostly carbon-dioxide) and these would be toxic for humans to breathe in. Earth is a unique planet with its ecosystem and people have adapted to these conditions on Earth. So going to another planet means that the environment is just different and we are not used to it. People are very sensitive to environmental conditions, for example, temperature, pressure, and so on. But we also know that there are small organisms on Earth (typically bacteria) that can survive very harsh conditions. These are called extremophiles. Some of these could, theoretically, live in conditions that are present on other planets and moons. People will most likely need to have safe space suits or closed stations to survive on Mars.