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If a carbon monoxide (CO) alarm goes off at work, leave the building for fresh air immediately. Do not look for the source or wait to see whether the alarm stops. Once outside, call 911 or your local emergency number, follow your workplace evacuation plan, and stay out until emergency responders or the responsible qualified authority says it is safe to return.
CO is colorless and odorless, and exposure can impair people before they realize what is happening. This guide applies U.S. federal guidance to workplace response, including construction and other sites where engines, tools, or partially enclosed work areas may create hazards. Follow your site’s emergency plan and use the local emergency number where you work.
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What should you do when the CO alarm sounds?
Evacuate first, contact emergency services from outside, and keep everyone out until the area has been assessed and cleared. The alarm is a warning of a potentially deadly hazard; do not try to confirm it by searching for a source or taking another reading yourself.
- Move to fresh air immediately. Leave the building and any affected enclosed or semi-enclosed work area. Use the evacuation route in your site plan if it is safe to do so. Do not delay to collect belongings, finish a task, or inspect equipment.
- Warn people as you leave, without slowing your exit. Alert nearby coworkers and direct them toward the exit if you can do so while evacuating. Do not go back inside to look for someone or to investigate. Tell emergency responders if you believe someone may still be inside or missing.
- Call 911 or the local emergency number from outside. Give the workplace location, explain that a CO alarm has sounded, and report any symptoms or known person still inside. Follow the dispatcher’s instructions.
- Get medical help for anyone with possible symptoms. Move the person to uncontaminated air and call for emergency medical help. Do not have someone who may be affected drive themselves to care.
- Keep people out. Do not re-enter because the alarm has stopped, a coworker thinks the space is safe, or an unverified detector reading appears normal. Wait for responders or the responsible qualified authority to assess the area and authorize entry.
- Follow the workplace emergency plan and responder directions. Once the immediate danger is controlled, the employer or site leadership can coordinate accountability, incident reporting, and the investigation needed before work resumes.
The U.S. Consumer Product Safety Commission’s CO alarm guidance says to move outside to fresh air, call emergency services, and not return until responders give permission. That page addresses household alarms; the same core emergency actions apply at work, alongside the employer’s site-specific evacuation procedure. The agency’s warning is direct: “Never ignore an alarming CO alarm!”
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What if someone has symptoms of carbon monoxide exposure?
Get the person into fresh air and seek urgent medical attention. Symptoms can be mistaken for a flu-like illness, but after a CO alarm they should be treated as a possible exposure—not as a reason to wait and see.
The CDC lists headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion among common symptoms of CO poisoning. NIOSH also identifies fatigue, seizures, and loss of consciousness. Severe exposure can lead to unconsciousness or death. Anyone with symptoms should be assessed by medical professionals, even if the symptoms seem mild or improve after leaving the area.
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Do not send a symptomatic worker back inside to retrieve equipment, shut down a machine, or assist with the response. Do not ask them to drive themselves to a clinic or hospital: impairment may affect their ability to drive safely. Call 911 or the local emergency number and follow the dispatcher’s directions. Tell responders where the person was working, when the alarm sounded, and what symptoms were observed, if that information is available without delaying care.
If more than one person feels ill, report that to emergency responders. Multiple people with symptoms can help responders understand the scope of the incident, but no one should re-enter to check on coworkers or determine the cause.
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Why can CO be a workplace hazard?
CO is produced by combustion, and some common work equipment can create exposure. NIOSH identifies portable generators, small gasoline engines and tools, marine engines, pressure washers, forklifts, and coal-burning power plants among workplace sources.
The risk is not limited to a sealed room. CO can build up rapidly inside buildings or semi-enclosed spaces, even where a space appears well ventilated. A work area that seems open or has airflow should not be treated as safe without appropriate assessment. That is one reason an alarm should trigger evacuation rather than an informal inspection by workers.
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On construction and industrial sites, equipment may move between indoor, outdoor, and partly enclosed areas as work changes. A generator or gasoline-powered tool used near a building opening can still present a hazard; NIOSH advises placing gasoline-powered engines outside and away from building openings. This is a prevention measure for planned work, not permission to approach equipment during an alarm.
Should anyone turn off equipment before leaving?
Do not delay evacuation to shut down equipment or find the source. NIOSH’s workplace guidance says workers with symptoms should immediately turn off equipment and go outdoors or to uncontaminated air. In an alarm response, only shut equipment down if it can be done immediately and without delaying or increasing risk; otherwise leave and tell responders what equipment may be operating.
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Workers should not cross an affected area, enter a building, or approach a suspected source to operate a switch. Do not ask a coworker to take that risk. The immediate priority is getting people to fresh air and contacting emergency services. Responders and qualified personnel can assess the area and determine how equipment should be handled.
When is it safe to go back inside?
Only after emergency responders or the responsible qualified authority has assessed the area and authorized re-entry. The alarm stopping is not clearance, and a worker’s sense of smell cannot help because CO has no odor.
NIOSH advises workers to stay away until equipment is deactivated and measured CO concentrations are below applicable guidelines and standards. Workers should not attempt to make that determination from a personal impression or an unverified reading. Follow the site’s re-entry instructions, including any limits on which areas may be entered and who is allowed to enter.
If the site has not communicated that the area is cleared, remain outside and ask the supervisor or incident lead for instructions. Do not resume work simply because a schedule is tight or the alarm has gone quiet. If anyone has symptoms, continue to prioritize medical care and tell responders that people may have been exposed.
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What should the employer or site lead do after evacuation?
After people are out and emergency services have been contacted, site leadership should support accountability and responders without sending workers back into the hazard area. The worksite should remain controlled until the area is assessed, the cause is understood well enough to address it, and re-entry is authorized.
- Account for workers from a safe location. Use the workplace’s established evacuation and accountability process. Report missing or potentially exposed people to responders; do not send a coworker inside to search.
- Share useful information with responders. From outside, communicate the alarm location, work area, equipment that may have been operating, and any reported symptoms. Do not guess at CO levels or identify a cause that has not been confirmed.
- Keep access controlled. Make clear that workers and visitors must not re-enter until the responsible authority gives permission. The alarm’s silence, a remote reading, or informal reassurance is not a substitute for clearance.
- Investigate sources and exposure conditions after the emergency. NIOSH recommends surveying the workplace for CO sources and monitoring employee exposure where CO may be present. Review where combustion-powered equipment was used and whether air movement, enclosure, or proximity to openings could have contributed.
- Reduce the chance of recurrence. Where safe and practical, consider electric, battery-powered, or compressed-air equipment as alternatives to gasoline-powered equipment. Place gasoline-powered engines outside and away from building openings, and test air regularly where CO may be present, as NIOSH advises.
- Train workers and revisit the response plan. Workers need to recognize the alarm response and possible symptoms, and know how to evacuate and whom to contact. Use the incident review to clarify roles and improve the site procedure without weakening the rule to leave first.
These are follow-up controls, not steps to perform before evacuation. No exposure assessment, equipment change, or investigation should delay getting people outside and contacting emergency services.
Do workplace CO monitors prevent an emergency?
Monitoring can be part of prevention, but a monitor does not change what to do when an alarm sounds: evacuate and contact emergency services. NIOSH recommends personal CO monitors with audible alarms where potential CO sources exist, and advises testing air regularly where CO may be present.
A personal monitor is a hazard-monitoring tool, not a substitute for source control, appropriate assessment, training, or the site emergency plan. This guidance does not identify particular models, alarm thresholds, calibration requirements, or certifications. Employers should determine appropriate equipment and monitoring procedures for their workplace and applicable requirements rather than relying on a generic product comparison.
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How serious is carbon monoxide poisoning?
CO poisoning can be fatal, and symptoms can progress from headache or nausea to confusion, loss of consciousness, and death. In the United States, the CDC reports more than 400 deaths, more than 100,000 emergency department visits, and more than 14,000 hospitalizations each year from unintentional CO poisoning not linked to fires.
Those are general U.S. estimates, not workplace-specific rates. They do not estimate the risk at any particular jobsite. Their relevance here is the seriousness of the hazard: a sounding alarm calls for immediate action even when no one has yet reported feeling ill.
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Leave immediately, call for help from outside, and keep people out until the area is cleared. CO cannot be seen or smelled, and a worker should not have to identify the source before taking the alarm seriously. For symptoms, fresh air and urgent medical attention come first. For employers, prevention means assessing sources, monitoring where appropriate, controlling combustion hazards, and training people to follow the evacuation plan.
Frequently Asked Questions
Can carbon monoxide from equipment outside get into a building?
Yes. NIOSH advises placing gasoline-powered engines outside and away from building openings, which is why “outside” alone should not be treated as sufficient separation. If an alarm sounds, evacuate and let responders assess the situation rather than trying to locate the source.
Does a CO alarm mean someone has already been poisoned?
An alarm signals a potential hazard; it does not by itself diagnose whether any individual has been exposed or poisoned. Treat it as an emergency, and seek medical evaluation for anyone with possible symptoms.
Are the CDC’s annual carbon monoxide poisoning figures specific to workers?
No. The CDC’s figures cover unintentional CO poisoning in the United States that is not linked to fires; they are not occupational statistics and do not describe workplace-only cases.
Can a personal CO monitor replace the site evacuation plan?
No. NIOSH recommends personal monitors with audible alarms where potential CO sources exist, but monitoring is a prevention and hazard-awareness measure. When an alarm sounds, workers still need to evacuate, contact emergency services, and follow site procedures.
