A HAZMAT suit is an overall garment designed to protect the wearer from hazardous materials. HAZMAT is an abbreviation for "Hazardous Materials", a US-specific term, and is elsewhere commonly termed "Dangerous Goods". Typically suits are worn with several other protective layers underneath, tape on the seals, and communications and medical monitoring devices inside. Usually, the suits must undergo an involved decontamination procedure before the user exits the suit after use. These procedures were never explicitly depicted.
Swan station occupant Kelvin Inman wore a HAZMAT suit when he ventured outside the station, including during his initial rescue of the shipwrecked Desmond from the beach. Later, Desmond observed that the HAZMAT suit Kelvin was donning was compromised with a large tear in the thigh, so he deduced that the concept of an outdoor pathogen, i.e. the sickness, was simply a ruse. He then decided to follow Kelvin outdoors unprotected in his normal clothing. The resulting confrontation resulted in the accidental death of Kelvin, and a system failure back at the Swan. Despite his conclusion that the sickness was fake, Desmond continued to take the vaccine CR 4-81516-23 42 and was surprised when he first met Locke, who told him that no one from the survivors of Flight 815 was sick. It seems highly likely Radzinsky used it once or twice.("Live Together, Die Alone, Part 1")
Charlotte and Daniel wore HAZMAT suits when attempting to render the toxic gas inert at the Tempest station. ("The Other Woman") These suits were delivered by airdrop and previously discovered by Jack and Kate (accompanied by Daniel). ("Confirmed Dead").