What is it about a Hazmat Suit?

Okay what is it about a 'hazmat suit', could it be protection, mask or boots all together or is it the colour yellow. (NB available in other colours lol ;-)) I have a few hazmat suits in my collection but yellow is my preferred colour! At the end of the day to a 'gear head' its about protection, looking the part helps. Matching yellow boots, suit and gloves, not forgetting the gas mask oh and the smell of all that gear ;-)