I ran tests on this stuff many months ago. Conditions can NEVER fall below 0.00. Conditions modifying events, facilities, etc., all add. There is never any multiplication (even in finite resources games). As stated before, conditions are stored in a variable that can only ever range from 0.00 to 1.50. You can never get a number lower or higher than this range. Any event that would drop or raise it out of this range instead drops it to 0 or raises it to 1.50, depending on the nature of the event (same for facilities). Planets with deadly conditions can most certainly be raised, it can just take a while.
About events... it would make sense that the value be divided by 100, so that an event with 20 becomes 0.20. This would effectively subtract 20 from the conditions value. However, it seems that the value instead might be getting divided by 10, which would make it drop the conditions value by 2.0, dropping it to 0.00 no matter what the starting conditions were. If this is the case, it seems like a bug to me.
