Frying oil smelling stale

I have read this Q&A before posting but could not find a more directly pertinent one.

I have some canola oil that was saved from frying a long time ago. I used the oil to deep fry some tempura back then and then put it in a container with the intention of reusing it for frying at a later time. I didn't get the chance to fry again for a long time until months later. Since the first time I used the oil I only used it for some light frying I expected it to be usable for a second time. However after half a year of storing in a cool and dark cabinet with no light exposure, the oil emitted a strong odor upon heating up. The best way I can describe the smell is it smelled stale. Not rancid like organic matters but smelled like industrial oil or old mechanic oil. The smell was so strong that it permeated the entire house and was not gone until a day later. Why is that? What is this smell? Also I was so confounded by this that I decided to heat up the oil again, but this time the smell is gone. Is this smell and oil causing this smell pernicious?

It is mainly due to a chemical process that happens when the fat is heated. The heat breaks down the oil's fats (triglycerides) into volatile organic compounds. This happens especially when you heat the oil to its smoke-point, so you should use an oil with a high smoke-point for frying.

How you handle the oil once you're done cooking and it has cooled has an impact as well. If you leave little bits of fried particulate in the oil, it will contribute to the smell.

Finally, oil breaks down over time whether you heat it or not. It naturally oxidizes over time (through reaction with oxygen in the air--even in an "air-tight" container) and can go rancid. (If you ever take a whiff of an old jar of peanuts, you'll smell a similar smell to what happened in your case.)

As for the smell being gone when you tried your "experiment", my only guess is that your house still stank, and you had become acclimated to the smell.

_{My sources for this answer was Google.}