It's good that you asked this becasue if it's to hot or cold in their environment they could die. The tempature I keep my guinea pigs in is 70-76 so the same tepature that a lot of people like. In the summer make sure to give them lots of cool water and cucumbers (they have lots of water) to your guinea pigs to cool them down.
Also, this is totally off topic but parsley is good to give the as it's high in vitamain C. Give them some twice or three times a week.
This is Twinkie (above) and Slinky (below) and they both seem to have a bad itch=)