I see this message popping up occasionally but don't know what it means in terms of game mechanics - I seem to be able to run around on the affected planets with no ill effects (I did see a blue bar appear above it on one occasion, but it appeared only briefly and never reappeared).

Level one hazard means that the toxins in the air aren't strong enough to kill you or deplete your HP. Your HP should never degen on Hazard Level 1 planets, only hazard level 2 and higher (If they even go higher).

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Yeah, pretty much. It's not all toxins, though. Hazard levels range from poisonous atmosphere to heavy surface pressure and heat. Level 1 doesn't do much but stop health from regenning, I think. The higher levels force you to only remain outdoors and on foot for a short period of time before losing HP.

im sure i got a level 3 one once and i think it was on a main mission