Computer covers have evolved to meet the demands of those using PCs and monitors in harsh, dusty conditions. Computer Dust Solutions has a breathable cover for the CPU, and a monitor cover made from the same breathable non-woven polyester material but with a clear, see-through vinyl front. These computer dust covers and monitor covers are designed to be used while the computer is running.
Regarding temperature increase, these breathable covers do increase the internal CPU temperature about 2 degrees C. Not too significant considering the increase in temperature and other damage dust can do to sensitive electronics.