Sorry for the late response. Reserves, civilian job, nightshift, blah blah blah, excuses!

Anyways, on to the answer!

They're actually rather different beings. Chlorine, which came first, chokes victims. It ultimately reacts with the water in the mucous lining in a victim's airway, creating hydrochloric acid. Depending on dosage, this can lead to death. Chlorine, however, is easy enough to deal with! It can often have a bit of a yellowish appearance and will usually come with a peppery and taste. All these show when chlorine is present. While a full protective mask is best, chlorine can be stopped via a simple wet cloth over the mouth and nose. It's non-persistent too! After a while, a contaminated area can be given an all-clear and demasking can begin.

Mustard, however, was made after Chlorine. Mustard is known as a Blister Agent. Whereas chlorine requires simply a mask for a reasonable expectation you'll walk away unharmed, mustard requires full MOPP 4. See, mustard is a skin irritant gone bad. This stuff is persistent for days, even weeks, (depending on concentration and temperature) and will persist on a victim's clothing and personal effects. Secondary exposure is possible through this persistence. The purest form of mustard gas is colorless and odorless, though it's often mixed with other chemicals and will then give off a garlic, onion or.. well.. mustard smell. It can be a vapor, oily liquid or solid.

Ultimately mustard gas exposure results in:
-Chemical burns to the skin
- Irritation of the eyes and possibly blindness
-Respiratory distress
-Diarrhea, vomiting and other discomfort

Reports from WWI suggest that mustard exposure is very~ painful. Luckily, however, exposure is not likely to be fatal to treated victims. With treatment, the biggest threat isn't the mustard, but complications such as infection or sepsis.

Much of this info is available online. The CDC is a good resource if you want to research more!

Signed:
Exceedingly Verbose Feathered Miniature Equine Customarily Designated "Vapor"
(I write too much)