If you're internet-dwelling losers like me, you probably know what SHUs are already. That is, unless you're a lonely internet-dwelling loser that doesn't give a fuck about science... in which case, shame on you. There's a wealth of information online that doesn't comprise midget porn, you sick fuck.
Anyway, here's the TL;DR on chile science for you.
We've previously discussed capsaicin and thyocianate, the evil motherfucers that turn your mouth into Satan's toilet and you colon into a heaping pile of pulsating blood clots, but we didn't delve too much on how they are meassured.
Scoville Heat Units were devised as units of measurement for his Organoleptic Test in 1912 by american biochemist Wilbur Scoville, who at the time was working for Boehringer-Ingleheim pharmaceuticals, the same company that makes Meloxicam.
An organoleptic test involves recording the results of a panel of testers, and being a subjective test, is highly imprecise. What Scoville did was to take the extract of every chile and dilute it with water, then feed it to a panel of 5 testers. A rating of 5,000 SHUs means that one drop of extract was diluted into 5,000 drops of water before it became undetectable by the tongue.
There's High Performance Liquid Chromatography (HPLC) available now, which is highly accurate in detecting the actual levels of irritant substances in chiles and spices, but 100 years of tradition have stuck and we rely on SHUs.
So what does this mean for us? Not much, actually, unless you want to search for a specific chile or hot sauce to suit your taste. Here's a great list of chiles and condiments that you can check before your next purchase.