The term “first responder” includes a firefighter, law enforcement officer, paramedic, emergency medical technician, or other individual, who, in the course of his or her professional duties, responds to fire, medical, hazardous material, or other similar emergencies.

First responders play a critical role in the diagnosis of carbon monoxide exposure.
In 2015 the Department of Health estimated that around 4,000 people attending Accident and Emergency departments each year in the UK are diagnosed with carbon monoxide poisoning.
Read more about the Department of Health study.
However, accurate numbers are difficult to determine because the signs and symptoms of carbon monoxide poisoning are very similar to other common illnesses and as such, are easily misdiagnosed.