Why is « stressés » plural in this sentence

In Lesson 14.5 which talks about the subjunctive, there is the sentence

Il est possible qu’on ait l’air stressés.

which in English is:  It is possible that we seem stressed.

Now in English, the adjective “stressed” is modifying “we”, but  isn't « stressés » modifying « l'air » ? 

I would have thought it should be singular.

The answer is fairly complicated, but you'll find an excellent treatment of the subject here:

https://yolainebodin.com/the-language-nook/french/avoir-l-air-followed-by-adjective

Thanks for that Robert.  Very imformative;I've bookmarked the site for the future!