Then move it. Air is the main source of cooling, but without it your PC will overheat. Move it out of any confined spaces and give it about a foot all around for proper air circulation. Dirt I've opened up some PC cases that were beyond disgusting. I've found ants, spiders, spider webs, twigs, leaves and dust bunnies the size of basketballs inside some PCs.
Dirt clogs up the few holes in a case that supply air and can cause fans to stop working.
Needless to say, my tool kit now includes a 10 horsepower leaf blower, an air-filtered face mask, rubber gloves and a gallon of Iodine. A lot of electricity flows through the CPU while it's working, and electricity means heat. The more and the harder it has to work, the hotter it gets. And with today's blazingly fast CPUs pushing the envelope more and more, heat is becoming a major issue. No matter what type of CPU your PC has, it needs a heat sink to help it stay cool.