When you take the lid off a skincare product jar, its contents are exposed to air and everything floating or dissolved in the air, like oxygen, nitrogen, air pollution, germs and dirt. Does air and its components spoil, inactivate, or in any other way adversely affect the structure or function of the skincare product?
The answer is easy. With two exceptions, which I'll explain later, most skincare products are not adversely affected by exposure to the air. Yes, there are all sorts of germs floating in the air like bacteria, fungus, and viruses, but skincare products that could be spoiled by them already have special ingredients called preservatives that prevent the growth of those germs, whether they fall out of the air or are introduced by your finger.
Nitrogen makes up more than ¾ of air, but you don't need to worry about it because it's inert... that means it doesn't react with or affect anything.
While the carbon dioxide and methane in air are greenhouse gases and together comprise less than 1% of air, they just don't react with the chemicals in skincare products merely by random contact with them.
Fine particles of dirt, dust and debris certainly can land in your opened jar of cream, but they are not apt to do anything more to your skincare product then they do to your skin when they land there... not pretty but harmless.
So it seems that opening the jar of your favorite skincare product doesn't doom it to instant decomposition or deactivation. But there are two things to consider.
If the product is water based, the longer the jar stays open, the more water evaporates from the product which can change the consistency or structure of the product to the extent that it no longer feels right, spreads right, or even does what it's supposed to do. So if you leave the jar open overnight, no big deal... cover it in the morning. But if you leave it open for a few weeks, it probably will feel, look or work differently.
The most important effect of aeration on skincare products is on antioxidants. When they are regularly and for prolonged periods exposed to oxygen and tiny amounts of ozone in the air, they can be inactivated and their color can even darken. That's why most products which are primarily antioxidants are not packaged in wide mouth jars. But don't worry, you don't need to stress each time you open your product as a little exposure won't do any harm.
So the good news is that simply reclosing your skincare products after use should prevent spoilage for all of them.