According to some people, the first of these sentences is perfectly fine, while the second has a common but nevertheless gutting mistake in it:
(1a) His romp through the woods was pleasant enough, but ending up in poison oak again aggravated his rash.
(1b) The constant itching over the next week left him quite aggravated.
That’s because these people believe that aggravate to mean “irritate” or “annoy” is a newfangled and improper meaning. To them, aggravate has but one meaning, and that is its earlier meaning of “to worsen”.
However, it turns out that neither of these are the original meaning of aggravate, which was “to make heavier”. In fact, this was the meaning with which aggravate was borrowed into English from Latin in the 15th century. That meaning has been all but lost in contemporary English — one can’t say “I aggravated the pick-up with my moving boxes” and expect people to make sense of that. So if you think you’re defending aggravate by sticking to its original usage, you’re doubly wrong.
Well, you might say, the “worsen” meaning is old enough, and the “irritate” meaning is still pretty new, so that’s why I’m against it. But that argument doesn’t hold water either; the first attestation of the “irritate” meaning in the Oxford English Dictionary is all the way back in 1611, only 15 years after the first “worsen” attestation.
The “irritate” meaning is a metaphorical extension of the original meaning; “make heavier” gets metaphorically extended to “add mental burdens” (think of other weight/thought/concern metaphors such as “weigh heavily on one’s mind”, “heavy thinking”, “weighing options”, etc.). From there it’s only a short hop to “irritate”, and we made it.
There’s no real loss in doing this. No significant ambiguity is gained by having these two meanings of aggravate. One can’t irritate an inanimate object, so (1a) is clearly using the “worsen” meaning. One can’t worsen a person, so (1b) is clearly using the “irritate” meaning.
There’re a few other complaints I’ve heard about the “irritate” meaning of aggravate. One is that there are too many words to mean “irritate” and not enough to mean “worsen”. But that presumes that the “irritate” meaning is eliminating the “worsen” meaning, and I don’t think that’s the case. Anyone who follows sports is familiar with the crushing feeling of their favorite player aggravating their existing injuries. Is this meaning losing ground? Probably. Might it disappear eventually, leaving behind only idioms like “aggravated assault”? Possibly, but that’s what language does. Why isn’t anyone shedding tears over the loss of a good word meaning “to make heavier”?
Another occasional argument I see is that aggravate meaning “irritate” is “less precise”, and I’m not really clear on what that’s supposed to mean. Less precise than using irritate? Irritate can also mean “inflame” (as in eye irritant) and has an obscure legal meaning of “make void”. The example I always give for how this “imprecision” problem is not a problem is mean. The word mean can mean so many things (average, grouchy, etc.), and yet we somehow encounter no problems from this lack of precision. In fact, the imprecision of language can be beneficial; compare your imprecise conversations with your friends to the precise language of your favorite legal contract.
The final objection is that aggravate meaning “irritate” is informal. (Jan Freeman discussed this in her column on aggravate.) This may have been the case at some point, but I am unconvinced that it is currently true. The Corpus of Contemporary American English provides various examples of this usage in modern formal academic writing, and Google Books can provide formal usages back into the 19th century. I suspect that the perceived informality is due the tendency of discussions of one’s feelings to be informal.
Summary: The “irritate” meaning of aggravate dates back to the 1600s, and it doesn’t interfere with the “worsen” meaning of aggravate. Neither is the original meaning of aggravate, either. There’s just no good reason to object to the “irritate” meaning.