Apple's written some suggestions to avoid overheating:
My suggestion is if you assume your machine is overheating, find out first if the firmware update hasn't been applied, if and it hasn't apply it. If the firmware update has been applied, run the hardware test which came with the machine. This articles you should find useful in that regard:
Here's the hardware test instructions for the Mac:
While this tip is primarily about MacBook and MacBook Pros, if you have a pre-2006 Mac with a PowerPC chip, the instructions for testing a PowerPC Mac for hardware faults are here:
Some Macs may have their hardware test available for download from:
Newer Macs than July 19, 2011 come with the hardware test prebundled with the restore partition. If you are able to boot with the D key and get to the hardware test, then you don't need to call AppleCare for the disc.
Backup your data frequently as hardware problems relating to heat could cause damage to the hard drive. Several methods of backing up can be recommended by people here on the board, and you should ask if you are unsure. If the hardware test finds the machine is having problems, call AppleCare:
It is also important to note the known EXTERNAL operating conditions that are deemed safe for Apple's machines which are stated in specs:
"Operating temperature: 50° to 95° F (10° to 35° C)
Storage temperature: -13° to 113° F (-24° to 45° C)
Relative humidity: 0% to 90% noncondensing
Maximum operating altitude: 10,000 feet
Maximum storage altitude: 15,000 feet
Maximum shipping altitude: 35,000 feet"
(there are 3.2808399 feet in a meter)
*The above temperatures and conditions are external ONLY! If you download*
*temperature monitoring software it should only be used to consider the internal temperatures, which are NOT documented by Apple. While you might be able to guess the temperature limit where failure might be possible, this is only guesswork, and only true hardware test software can tell if the failure is actual failure, or just arbitrary guesswork.*
Warning: Some who are technically inclined have tried modifying the thermal paste, which if fails may lead to a billed repair. I strongly recommend you don't do this. In addition, some have tried to use software that modifies the fan operation. Unfortunately this too is risky, as it assumes you know when the machine is going to overheat and when it is not. And you would not want to be wrong on this, as these machines can get quite hot if tampered with the wrong way.
Some may add to their notebook a shell to protect it against falls. Such a shell may insulate heat around the notebook computer, causing the machine to overheat, and unexpectedly shut down, or freeze while it attempts to cool itself down. If you are using some exterior wrapping on your machine and are experiencing such symptoms when using the machine while that wrapping is on it, remove the said wrapping carefully over a soft surface that won't damage the machine in event it slips out of your hand, and see if the issues with unexpected shutdown persist. If they don't, it says the manufacturer of the said shell needs to offer better ventilation for the computer when it is in use.
Additionally, some may travel with their machine asleep. Alas, some software is known to wake Macs in the middle of their sleep even with the screen shut tight. While it may take longer, shutting down your machine prior to travel will avoid such a scenario from happening. Not to mention, if such a scenario were to happen, your battery charge could become much less, as the screen would be on for the entire duration of time from the time it wakes up.
Lastly, some hardware issues come out as part of an exchange/repair program:
If your machine falls in one of those categories, it might fix an overheating issue to get it repaired under that program.