Unambiguously referring to "spiciness"

Spiciness is a taste perception, and as such, it is simply subjective. There is no way to create an objective scale for rating it.

I read your comment about "objective spiciness", but it is not something that can exist. It is based on the erroneous assumption that the spiciness you perceive is a 1:1 measurement of some quality of the food. This is not true. Even foods with the same amount of capsaicin will taste differently spicy to the same person, or the same food can taste differently spicy to the same person in two separate meals. So no, all scales will be subjective, with all the usual consequences of subjective rating scales.

The best choice, in my opinion, is not to try to communicate - but to ask for a taste! Places that use spicy sauces will likely be willing to give you a small taste of the sauce (as spicy sauces are usually made in advance, and even if not, they likely prepare the dish for another person at some point).

And even if you're asking about something that isn't really tasteable ("How spicy are your spicy tuna maki") etc., you could ask for a taste of a sauce that is spicy, and then use that to calibrate.

I don't think there is an unambiguous system -- for example, the Thai places in my town alone have vastly different systems for rating how hot their dishes are (so an extra hot at one place is a medium at another, for example; this may be partially due to location and thus their clientele). It isn't like restaurants do some sort of analytical measurement on their food's spicyness, since it varies depending on ingredients (sometimes your peppers are mild while others are blazing hot), so it will likely be calibrated to what the local population tolerates. Plus, even if there was a system more complex than "no spice, mild, medium, medium hot, hot, extra hot" or equivalent in number of chili's, its not like the wait staff or chefs would be able to keep up with it in a reasonable fashion.

In some cases, you can order food without heat (or on a lower heat level which you believe will be acceptable to you) and get the spicy stuff on the side (e.g. in the form of some chili sauce). Then, you can add it to some appropriate level for you after trying the food. It might not be as flavorful as if it was cooked in the dish, but by setting the bar appropriately, you should be able to reach something tolerable.

No, there is no objective way, other than the Scoville scale or measuring the moles of capsaicin per gram, because people lose sensitivity to spiciness as they get used to it. I would describe how spicy you want something by the type of pepper you are used to. If you can eat a habañero, then tell them you want it habañero spicy.

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As Rumtscho has pointed out, the perception of spiciness is strictly subjective. But if you were to have a standard, how would you standardize it? How would the chef comply with the standard? And then, when you get what objectively conforms with the standard you requested, how would you know you've got it? Above all, would you like the taste?
First, standardizing it: you take some scale. This would reduce to an objective measurement of capsaicin, and whatever other compounds contribute to a spicy taste. Well and good. Who standardizes each pepper? Two peppers identical in appearance and type might have widely different concentrations of capsaicin. Ground spices will be even worse. Where I live, one of the problems of buying spice in the market is knowing which are adulterated with salt or corn meal to bulk them up.
Good chefs go by the taste of the food as they prepare it. This applies particularly to spicy cooking -- the idea of measuring spices out in milligrams would not occur to a good chef, because the final result would be all that counts.
When you finally get your standard, calibrated, weighed and measured spicy, the taste will vary according to what's cooked with it, what you've eaten before, what you're eating after, whether you're having alcohol with it -- any number of factors. Then what does your standard count for?
What you really want is to enjoy your food. If you're cooking for yourself, take it seriously, listen to the food, taste from time to time to make sure you're heading in the right direction. If you're eating in restaurants, find one that does things the way you like, and be prepared to be delighted sometimes and, rarely, a little disappointed. And if you're eating at your mother-in-law's, always thank her with a smile and a little flattery, but not too much, so you all enjoy the meal!

There is no way to have this conversation on a consistent basis without ambiguity. You may find you could at certain restaurants who understand you well, but you'll never find a one size fits all way of communicating affinity for spice usage.

It's because the two variables have tremendous variance in the population:

• the diner's individual tolerance and labeling for spice level varies tremendously
• the cook's labeling of his/her own spice use will vary as well

Example: I can eat very spicy food and not even notice it. My dad would sample from the same dish and would revolt at the spice level. But we both would have said I like it medium spicy.

And you are correct, Scoville does not address your Q. Scoville addresses pungent heat. I consider heat (or pepperiness) to be a subterm of spicy. There are other attributes of spicy that are not pungent heat, like some curry's, level of garlic and onion, and use of herbs.

I tried to think of a way to communicate spice tolerance and altering this chart from the medical world came to mind:

But as I said before, my Dad and I would self label as a 5, but he clearly has lower tolerance than I.

While 'hotness' is somewhat subjective, most people who cook with spices understand the concept of adjusting the heat level according to the palate of others. Simply saying "mild", medium, spicy, or slap your mama my face is on fire! The other thing to do is to just take the ride and eat it the way it is prepared. One proven way to cool the heat is with dairy try so have some milk on hand if you know you're going to be eating spicy food.