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

The presence of delayed onset blisters combined with a dry, productive cough that later becomes more pronounced points towards mustard gas as the likely culprit. Mustard gas, known as a blistering agent, does not typically cause immediate symptoms. Instead, individuals exposed to it may experience a latency period before signs of pulmonary and skin damage become apparent.

The blistering effect is caused by the chemical's ability to damage the skin and mucous membranes, leading to the characteristic blisters. Furthermore, the respiratory symptoms, including a dry cough that may progress to becoming productive, align with the pulmonary irritative effects of mustard gas as it affects the respiratory tract.

In contrast, while other agents like sarin and phosgene do have destructive effects, their symptom presentations differ. Sarin is a nerve agent causing acute symptoms, and phosgene is a choking agent that would lead to immediate or early cough and pulmonary edema rather than a delay in blister formation. Chlorine gas can cause immediate respiratory distress but typically does not result in blistering as seen with mustard gas. Thus, the symptoms outlined in the question align most closely with the known effects of mustard gas exposure.