MSFinals-5800

A 60-year-old man visits his General Practitioner complaining of shortness of breath, nocturnal cough and wheezing for the past week. He reports that these symptoms began after he was accidentally exposed to a significant amount of hydrochloric acid fumes while working in a chemical laboratory. He has no prior history of respiratory issues or any other relevant medical history. He is a non-smoker.
What initial investigation may be the most useful in confirming the diagnosis?