Published 27 June 2026 • By Dr. Megan Tranter
Carbon monoxide kills quietly. It has no color, no odor, and no irritant warning, so by the time a worker recognizes the headache and confusion of poisoning, judgment and escape capacity are already impaired. It remains one of the most common causes of fatal occupational and accidental poisoning worldwide, and almost every case is preventable with recognition of sources, good ventilation, and continuous monitoring.
What You’ll Learn
- Why carbon monoxide is a chemical asphyxiant and how carboxyhemoglobin impairs oxygen delivery
- The OSHA PEL, NIOSH REL and ceiling, and ACGIH TLV for carbon monoxide
- The NIOSH IDLH of 1,200 ppm and what it means for confined-space entry
- Common occupational sources and the warning signs of poisoning
- Controls and monitoring strategies that prevent CO exposure
Introduction
Carbon monoxide (CO) is produced by the incomplete combustion of carbon-containing fuels. Internal combustion engines, gas and propane forklifts, furnaces, kilns, gas water heaters, welding and cutting, charcoal, and the use of explosives all generate it. Because CO is a simple molecule that diffuses readily and provides no sensory warning, it is especially dangerous in enclosed, poorly ventilated workplaces. Understanding its mechanism of toxicity explains why even modest airborne concentrations, sustained over a shift, can produce serious harm.
Mechanism: A Chemical Asphyxiant
Carbon monoxide is classified as a chemical asphyxiant because it interferes with the transport and utilization of oxygen rather than simply displacing it. CO binds to hemoglobin with an affinity roughly 200 to 250 times greater than oxygen, forming carboxyhemoglobin (COHb). This has two effects: it directly reduces the oxygen-carrying capacity of blood, and it shifts the oxyhemoglobin dissociation curve to the left, so the hemoglobin that still carries oxygen releases it less readily to tissues. CO also binds to myoglobin and to cytochrome c oxidase, impairing cellular respiration directly. The organs most sensitive to hypoxia, the brain and heart, suffer first.
Symptoms track roughly with COHb saturation. Background COHb in non-smokers is typically 1 to 3 percent, and in smokers it may reach 5 to 10 percent. Headache and mild exertional breathlessness appear in the range of about 10 to 20 percent; throbbing headache, confusion, and impaired manual dexterity appear in the range of about 30 to 40 percent; and loss of consciousness, seizures, cardiac arrhythmia, and death appear at higher levels. Because the early symptoms (headache, nausea, dizziness, fatigue) mimic viral illness, occupational CO poisoning is frequently misattributed, which is one reason monitoring matters.
Exposure Limits
The OSHA permissible exposure limit (PEL) for carbon monoxide is 50 ppm as an 8-hour time-weighted average (29 CFR 1910.1000 Table Z-1). The NIOSH recommended exposure limit (REL) is more protective at 35 ppm as a TWA, with a ceiling of 200 ppm that should never be exceeded. The ACGIH Threshold Limit Value (TLV) is 25 ppm as an 8-hour TWA. ACGIH has placed carbon monoxide on its 2026 Notice of Intended Changes, signaling that a revision to the TLV is under review, but 25 ppm remains the current adopted value until any change is formally adopted. The NIOSH immediately dangerous to life or health (IDLH) value for carbon monoxide is 1,200 ppm; above this concentration, escape may be impossible without respiratory protection, and entry requires the most protective supplied-air equipment. For confined-space work, atmospheric testing must confirm CO concentrations are below action levels before and during entry, because engines, hot work, and decomposition can rapidly drive concentrations into the IDLH range. These principles connect directly to confined-space and gas-and-vapor exposure management discussed in the related posts below.
Sources and High-Risk Tasks
Recognizing sources is the first line of defense. Propane and gasoline forklifts operated indoors are a classic cause of chronic low-level poisoning in warehouses. Portable generators and pressure washers used in enclosed or partially enclosed spaces have caused numerous fatalities. Gas-fired space heaters and furnaces with inadequate venting, blocked flues, or cracked heat exchangers release CO into occupied areas. Welding, brazing, and the use of explosives in tunneling and trenching can produce dangerous concentrations, and CO has been documented migrating through soil into confined spaces after nearby blasting. Vehicle exhaust in repair shops, idling trucks at loading docks, and ice-resurfacing machines in indoor rinks are recurring problem areas. Any task that combines combustion with limited ventilation should be treated as a CO risk until proven otherwise.
Controls and Monitoring
Prevention follows the hierarchy of controls. Where feasible, eliminate indoor combustion sources by using electric forklifts, electric tools, and other electrified equipment. Where combustion is unavoidable, engineering controls, principally dedicated local exhaust (such as tailpipe extraction in vehicle bays) and adequate general ventilation, keep concentrations low. Routine maintenance of engines, burners, flues, and catalytic converters reduces CO generation at the source. Continuous CO monitors with audible and visual alarms, both fixed in high-risk areas and worn by workers, provide warnings that human senses cannot. Alarms are commonly set with a low alert near the TWA and a high alert well below the IDLH. Personal protective equipment, specifically supplied-air respirators or self-contained breathing apparatus, is reserved for emergencies, rescue, and entry into atmospheres that cannot be made safe, because air-purifying respirators do not protect against CO. A complete program also includes worker training to recognize symptoms, evacuate, and seek medical evaluation, where 100 percent oxygen or hyperbaric oxygen therapy may be indicated for significant exposures.
Summary
Carbon monoxide is a colorless, odorless gas that is a chemical asphyxiant that binds to hemoglobin to form carboxyhemoglobin, starving the brain and heart of oxygen. The OSHA PEL is 50 ppm, the ACGIH TLV is 25 ppm, the NIOSH REL is 35 ppm with a 200 ppm ceiling, and the IDLH is 1,200 ppm. Because workers cannot detect it, continuous monitoring with alarms, combined with the elimination of indoor combustion and good ventilation, is the foundation of CO safety.
Helpful Resources
- NIOSH Documentation for IDLH Concentrations: Carbon Monoxide
- NIOSH Pocket Guide to Chemical Hazards: Carbon Monoxide
- OSHA Carbon Monoxide Safety and Health Topics
- Related posts: Exposure to Gases and Vapors and Confined Spaces.
Bibliography
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