Putting your computer inside the fridge isn't necessarily a problem by itself, but it is no solution either. But first, I do have a few comments beyond what I already read in other answers.
- Putting the warm laptop in a plastic bag or sleeve before placing it into the fridge is a bad idea. Warm air holds lots of moisture; as it cools, you will see condensation inside the bag, which will affect the computer.
- As others noted, when you take out the computer, parts of it that are cold may see condensation from ambient air.
- Putting the machine in the fridge does not remove the root cause of the overheating. It just puts more thermal stress on the computer by increasing the temperature difference between its hot parts inside and its outer surfaces or the air circulating through it.
But before resorting to using the fridge, I would first investigate why your computer overheats. Because it really shouldn't. You certainly shouldn't have a CPU running at 90 °C, throttling so badly that it misses keystrokes.
First, is the hardware really healthy? I don't have a MacBook Air, but looking at images of it online, it seems that the CPU fan vents directly to the outside, so presumably you can tell that it is running. Just be sure, because if the laptop has more than one fan and the CPU fan itself is dead or dying, other fans may be doing overtime but that won't really help.
Beyond that, I'd look at the software. You say you are a "heavy user" and you describe an office environment. What does that mean? Because most office activities consume very little CPU power. Even development tools consume little power except when you are recompiling a project, for instance, but those activities typically don't take too long even for complex projects. If you are working on documents, spreadsheets, diagrams, etc., with low CPU activity, your nice i7 CPU (I looked it up, a 2013 MacBook Air Pro with a 2.3 GHz i7, that would mean an i7-4850HQ with a thermal design power of only 47 W) should stay cool as a cucumber (from what I am reading online, the CPU temperature should in in the mid to high 40s Celsius when idle; a little higher, but certainly no more than 50-60 °C under a light load).
If the CPU doesn't stay cool and its fan is working properly, that means that something is using a lot of cycles. Typically, that something would be, say, a demanding computer game; a complex numerical calculation; video rendering; or just some badly written software (Microsoft is notorious for some of its components like Windows Update or the search indexer, consuming tons of CPU cycles for extended periods of time; I don't know so much about Apple). Check, if you can, what is using those CPU cycles. Is it something you can live without? Uninstall it. Can it be throttled? Is it misconfigured?
The bottom line is that unless you are doing something that is really CPU intensive, your machine should stay cool on its own, not much warmer than the ambient temperature, with its fans spinning at a low RPM. Anything higher than that, you know that something on that computer is not behaving properly. And in the long run, no matter what method you use to cool that computer, extensive thermal stress is going to cause it harm.
