Timing is everything. You can get quite close to the vents without being burned. As soon as the steam subsides, run past--but watch for additional steam discharges in the way. In some places you will have to turn a steam vent off by switch to get past it. In other areas you can break open steam pipes (to your disadvantage) by shooting or tossing charges at the wrong spot. Steam doesn't affect enemies, so leading them through it is a waste of time. Get scalded and it'll cost you 5 points.
You'll only see these on a couple levels. Same deal as steam vents, but the stakes are higher: 5-10 health points.
You will only encounter these on Level 154 (ATMOSPHERE PROCESSOR). Your presence on the grates will cost you 5 health or armor points per second. You will need to decide if the powerups down there are worth the loss.
These only show up on Level 232 (FURNACE CONTROLS). Move very carefully, since getting too close knocks your health or armor down 5 points. Watching out for the eggs and adult dogs in this area at the same time doesn't make it easy!
You can fall or drop quite a distance before it will hurt you. Most of the harmful drops will cause you to lose 5 health points, but farther drops will cost you 15 points or even your life. Armor does no good here. Be careful not to get stuck on a rising pedestal or lift that will require a painful drop! Getting left high and dry by the untimely activation of a lift that you needed to stay in the "up" position will also yield the same results. Despite your best efforts, however, some harmful falls in the game will be entirely unavoidable, especially if you want to discover all of the secrets!
This comes in two forms: alien acid, and acid pools (or puddles) on the alien ship. First, alien acid. As mentioned before, on HARD a resurrected alien will leave an invisible acid puddle where it was last killed. The only way to avoid this (outside of acquiring a photographic memory) is to try to drop the critters to the sides of corridors and away from doorways whenever possible; another trick is to drop multiple aliens in the same exact spot. Both these tips are also a good idea most of the time, anyway. The only alien that doesn't leave an acid spot when killed is the egg, but any alien vaporized by a direct seismic charge or grenade hit will also leave no trace of acid behind. Try to skirt acid puddles on the alien ship--although there is at least one spot where it is entirely unavoidable. The only way to avoid taking damage in acid pools (and you WILL have to wade some) is to don the all-too-scarce acid boots. Clear your path of enemies as much as possible beforehand, because you will need every second of protection the boots provide once you grab them.
Detonate any of these at close range and you'll be sorry. Look for more detailed information under WEAPONS and OTHER ITEMS.