For Aron: AC on ISS

Aron asked: One a scale of 1 to 10, how bad would it be if the ISS lost it's AC?

The ISS doesn't have air conditioning the same way we have here on earth. On earth, air is moved in and out of homes here, but is just "conditioned" by being cooled and dehumidified.

On orbit, it's a bit more complicated.

The Environmental Control and Life Support System removes CO2, adds oxygen, regulates pressure and O2 partial pressure, removes trace contaminants, dehumidifies the air (while recycling the condensate into the water supply) and regulates temperature.

How important is each of these capabilities on the ISS on a scale of 1 to 10? - 10

If the system fails to remove CO2, the crew will die.
If the system fails to add sufficient oxygen, the crew will die.
If sufficient cabin pressure and partial pressure of oxygen, the crew will die.
If trace contaminants are not removed in the fully enclosed environment, the long term health of the crew can be at risk.
If the air is not maintained at the right humidity, condensation is a serious concern, especially in an environment with electronics everywhere, the electronics that keep the system oriented and powered and everything else - too much humidity and the crew and ISS is at risk. Many electronics systems are on cold plates for cooling - making condensation a real issue.
If the air is not at the right temperature, electronics can fail (we don't have any but forced convection on orbit since there's no gravity). Too hot, and systems overheat and fail. Too cold, and, again, you have condensation issues.

Most of these systems have redundant components and subsystems because they're so key.