A group of people is presenting with a delayed onset of blisters and a dry unproductive cough that later turns productive. what is the likely culprit?

I believe the answer is: Mustard

Sulfur Mustard is known to be a powerful irritant, which would damage our skins and organs if it got into contact with them. Irritation could also happen if mustard somehow got into our pipes, which might cause the dry unproductive cough and increase in severity over time.