Modern helmets are all synthetic and plastics and should be able to withstand reasonably careful washing, even complete dunking in cool or lukewarm water. I wouldn't use hot water, and it's probably unnecessary. The only time I've seen separation between the plastic shell and interior EPS foam was after my noggin smacked the pavement when I was hit by a car -- the thin, harder plastic outer shell peeled away from the EPS foam at the point of impact. But hot water might accelerate separation.
I hand wash most of my cycling and running kit, so once in awhile I'll fill a 5 gallon bucket with warm water and clothes detergent (usually free of dyes, softener, etc.), stuff in my stinky jersey, etc., and set the helmet on top of the bucket with the straps dangling in the water -- and toss the detachable pads into the bucket. Let 'em soak for at least 20 minutes.
After soaking I'll use a sponge to wipe down the straps (move the buckles and Y-splitters to get the crevices), inside and outside the helmet, rinse under the tap, and hang to dry. I'll clip the padding to a mini-clothesline over the tub.
Years ago when I drank only plain water during bike rides I rarely needed to wash my helmet and never saw any crusty salt marks. But after adding electrolytes to at least one bottle on every ride I sweat out a lot of that stuff and get salt frost on everything I wear.