Phase Change Materials (PCM) are substances which absorb or release a large amount of "latent heat" when they go through a change in physical state ie., from solid to liquid or vice versa. During this change of phase, their temperature remains constant. This property of PCM is utilised in personal cooling products to offer constant temperature cooling. PCM technology works in high or low humidity situations and requires no air flow. This makes PCM vests perfectly suited for use use under heavy workwear, HAZMAT suits, military or fire fighter gear, mascot costumes etc. The PCM used is usually paraffin, bio based, salt hydrates or pure water. These vests are also referred to as ice vest, gel cooling vest, gel pack vest, AC vest, cool jacket etc.