I expect that you have already considered the most probable answer. For some spices, the aromatic compounds are released by some heat, but are not destroyed. Thus their effect is heightened. For other compounds, even a small amount of heat will destroy them and thus reduce their effect - see Mischa's comment.
I believe that the difference essentially lies in at what temperature a flavoursome compound starts to degrade.
There are indeed different varieties of sorrel,* but they're both/all high (though I can't determine how high) in the substance of concern, which is oxalic acid. This acid is found in lots of other green leafy vegetables, notably spinach and parsley, and is the reason that you shouldn't eat rhubarb leaves.**
It seems to have a reasonably high expected lethal dose; the internet echo chamber puts it at 600 mg/kg, which would mean eating pounds and pounds of whatever leaf you've got. The necessary amount to cause sickness is obviously much lower, however. There's also a long-term risk for kidney stones -- oxalic acid combines or reacts with calcium to form calcium oxalate, which is a major component of those stones (and this is the mechanism for more immediate poisoning, whether lethal or not). I have also seen reports of simple mouth and digestive irritation, again caused by the calcium salt.
Unfortunately, it seems that cooking does not destroy oxalic acid. It may be possible to reduce its presence by boiling and discarding the water, however. It's unclear to me whether this is because the acid itself dissolves into the water, or whether the oxalates (Harold McGee says that the potassium and sodium salts are water-soluble) form during cooking and are dissolved.
In all, I think the only recommendation that can be made is simple moderation. It's incredibly unlikely that you'll kill anyone with a salad, but don't eat the stuff in large amounts every day.
*"Common" (looks like elongated spinach) and "wood" (looks a bit like clover, but the leaves are cardiod) are the two that I'm most familiar with.
**Although it seems that there's possibly at least one other poisonous substance in the leaves.
Best Answer
I get the same situation repeatedly when preparing stir fry. Fragrant dry spices (like red pepper flakes) intensify in flavor when roasted in hot oil for a short (!) amount of time, lest they burn.
To circumvent the peppery air that can be quite unpleasant if you're not used to it, you could:
But I am not aware of anything that can really draw the peppery vapors out of the air in your kitchen.