In winter you can put on a coat to keep warm.  The coat works by retaining your body heat. 

But what can you do during summer to stay cool? Well, think of a cool vest as a coat for summer.  Cool vests use advanced materials that undergo a phase change at comfortable temperatures. 

This phase change is similar to ice changing to water and absorbing a lot of heat in the process.  The difference is that new materials have been developed that undergo this change at a more comfortable and efficient temperature.

Cool vests are an efficient solution to the cooling problem. Instead of cooling the air around you, the cool vest cools you!

 Cool vests that use ice or chemicals have been around for a long time.  They do increase the amount of work you can safely do on a hot day.

The problem is that they don't do much for your comfort level. Vests that use ice or colder chemicals are very uncomfortable and give a strange too hot/too cold at the same time feeling. Also, colder vests waste more energy by cooling the air around you more and your body less.  

The best cool vests I've found are from Glacier Tek. 

Glacier Tek cool vests are much more comfortable than other vests because they maintain a nice, comfortable 59°F. Most competitive products are from 32°F-40°F. 

In a jobsite environment, cool vests are very cost effective because they not only make you more comfortable they increase the amount of work you can do by over 30% when it is hot.