What COHb represents
Carboxyhaemoglobin is haemoglobin bound to carbon monoxide. This binding impairs oxygen transport and availability. Blood measurement can provide information about the internal dose following exposure. It is not equivalent to measuring CO in workplace air: the two measurements answer different questions, and neither should be interpreted without knowing the circumstances that produced it.
Relevant workplace situations
Testing may be relevant where combustion, engines, furnaces, fires or inappropriately used petrol equipment create exposure. Investigation needs to identify the source, ventilation, tasks and duration. Biological exposure monitoring complements this assessment. It does not replace environmental detectors and cannot be used to authorise entry into a hazardous atmosphere or certify that a space is safe.
Time changes the result
COHb can decrease after a person leaves contaminated air and is affected by treatment received. Record when exposure ended, when the sample was collected and whether oxygen was administered. An apparently low result obtained later cannot, by itself, exclude an important exposure several hours earlier. The analytical result therefore needs a reliable chronology of the incident and subsequent care, including earlier tasks and any known equipment failure during the shift.
Other sources and influences
Smoking can increase COHb and must be considered alongside domestic or environmental sources. There is also endogenous production, and metabolism of substances such as dichloromethane can generate CO. These factors do not justify dismissing workplace sources; they require a more complete exposure history. An increase should not automatically be attributed either to the job or to smoking.
Why an ordinary pulse oximeter is insufficient
Conventional pulse oximetry does not adequately distinguish carboxyhaemoglobin and may provide false reassurance. Co-oximetry uses a different analytical approach to identify haemoglobin fractions. The COHb percentage also does not have a simple relationship with severity or prognosis. Clinical assessment combines symptoms, history, examination and exposure information; one numerical result cannot determine the whole clinical picture.
Monitoring and emergencies differ
A preventive programme specifies sampling and interpretation through current references and clinical protocols. Suspected poisoning requires the appropriate emergency response and prevention of further exposure, without waiting for scheduled monitoring. Subsequent investigation reviews equipment, extraction, maintenance and alarms. Clinical findings remain confidential, while necessary workplace measures are communicated to the people responsible for implementing them and checking that they work.
