Carbon monoxide is a toxic, odorless, colorless gas created by the combustion of carbon-based fuels (gasoline, natural gas, propane, wood, oil). It can originate from furnaces, fireplaces, water heaters, portable generators, attached garages, and even mis-vented appliances like a dryer or chimney. Because CO cannot be seen or smelled, it’s widely called the “silent killer.” A properly listed residential CO alarm alerts before exposures become life-threatening per UL 2034 response criteria. If your alarm sounds, get fresh air immediately, call 911, and seek medical care—CO binds to hemoglobin with ~200–250× the affinity of oxygen, forming carboxyhemoglobin (COHb) that impairs oxygen delivery; COHb has a half-life of ~4–6 hours on room air, ~60–90 minutes on 100% oxygen, and ~20–30 minutes with hyperbaric oxygen at 2–3 ATA (CDC clinical guidance; StatPearls). Vulnerable groups (including pregnancy) are at higher risk and require lower thresholds for concern (CDC). For prevention tips and placement, see the CPSC Carbon Monoxide Information Center.
What Is a CO Alarm?
A CO alarm is a life-safety device that detects carbon monoxide and warns occupants via a local sounder and (in many models) connected notifications. In homes, single- or interconnected alarms are listed to UL 2034, which standardizes alarm behavior and timing at specific CO concentrations. System-connected detectors used with control panels in buildings are listed to UL 2075 and can trigger ventilation or notify responders. Installation, testing, and maintenance requirements for system-based CO detection are covered in NFPA 72 (2025 edition) after CO provisions were consolidated from the withdrawn NFPA 720. Smart-home options increasingly offer standardized states and alerts across brands through Matter’s Smoke/CO alarm device type (CSA: Matter 1.4), and some ecosystems can coordinate safety actions like pausing HVAC to help limit CO spread (Nest Safety Shutoff).
Carbon monoxide levels, referred to as “ppm” or “parts per million”, indicate how many CO molecules are present per million parts of air (by volume). For general definitions and unit context, see is equivalent to. Residential alarms listed to UL 2034 are intentionally designed not to alarm at very low concentrations to reduce nuisance activations, and they must respond within defined times at higher levels: typically 70 ppm (60–240 minutes), 150 ppm (10–50 minutes), and 400 ppm (4–15 minutes) (First Alert CO400 manual).
Where to put a carbon monoxide detector in your home?
The in a hallway outside sleeping areas is a common location so alarms are audible at night. As a rule of thumb, install CO alarms on every level of your home, including the basement, and outside each sleeping area per safety guidance (CPSC; USFA). The 2024 International Residential Code requires CO alarms in dwellings with fuel-burning appliances or attached garages and specifies UL 2034 for single-/multi-station alarms and UL 2075 for system devices; new construction generally requires permanent power with battery backup and interconnection (IRC R315). Follow the manufacturer’s instructions for mounting height—CO mixes with room air; if your model recommends installing at least 5 feet high or on the ceiling, follow that guidance. Avoid locations that can cause nuisance or delayed detection: do not mount directly next to fireplaces, furnaces, or fuel-burning appliances; keep away from bathrooms and supply vents; and avoid garages (use proper sensors as part of a system). In areas with potential hydrogen (H2) presence—such as attached garages or charging areas—choose products that specify reduced H2 interference to minimize false alarms (H2‑compensated electrochemical sensors). For additional do’s and don’ts, see CPSC Q&A.
What safe and unsafe levels look like
- 0–50 ppm – Short exposures at these levels may produce few symptoms in healthy adults, but sensitive groups (pregnant people/fetuses, children, older adults, those with heart or lung disease) can be affected. WHO guideline values for indoor air target much lower averages to protect health (e.g., 8-hour ~9 ppm; 1-hour ~31 ppm) (WHO). UL 2034–listed residential alarms are designed not to sound at very low concentrations; models with displays may show low ppm without alarming (UL 2034 response behavior). If low-level readings persist, service fuel-burning appliances and improve ventilation.
- ~50–150 ppm – Action required. OSHA’s permissible exposure limit is 50 ppm as an 8‑hour TWA; NIOSH’s recommended exposure limit is 35 ppm (8‑hour TWA) with a 200 ppm ceiling; IDLH is 1200 ppm (NIOSH Pocket Guide). UL 2034 requires alarms between 60–240 minutes at 70 ppm and 10–50 minutes at 150 ppm (manufacturer criteria). Headache, nausea, and dizziness can occur within 1–2 hours at these levels. Evacuate, call 911, and breathe fresh air; medical teams typically administer 100% oxygen, which shortens COHb half-life to ~60–90 minutes (CDC).
- 150+ ppm – Dangerous, seek immediate medical attention. UL 2034 alarms must sound rapidly at higher concentrations (e.g., 400 ppm within ~4–15 minutes) (UL 2034 timing). Severe symptoms include confusion, syncope, chest pain, seizures, coma, and death (U.S. Consumer Product Safety Commission; StatPearls). In the ED, clinicians consider hyperbaric oxygen for COHb ≥25% in adults (≥20% in pregnancy), loss of consciousness, severe neurologic symptoms, ischemic ECG/troponin changes, severe acidosis, or persistent symptoms despite high‑flow oxygen; HBO can reduce COHb half-life to ~20–30 minutes (CDC).
Guide to Your CO Detector
Do:
- Read and follow the installation instructions from the manufacturer and verify the device is listed to the correct standard (homes: UL 2034; system-connected: UL 2075). When part of a fire/combination system, follow NFPA 72 (2025) for installation and testing.
- Test the alarm monthly and replace it at end-of-life (commonly 5–10 years with an EOL chirp). Night-time chirps are a common consumer complaint but are an important safety signal—replace the unit when it indicates end-of-life (USFA guidance).
- Install multiple CO detectors if you have a large home—place them on every level and outside each sleeping area for audibility and coverage (see IRC R315 and CPSC).
Don’t:
- Place your CO detector where the furniture or curtains block it from adequate airflow—keep devices clean, unobstructed, and per the manufacturer’s spacing from corners/“dead air” zones.
- Place your CO detector in your kitchen or garage—cooking byproducts and vehicle exhaust can cause nuisance activations or delayed detection. In garages or charging areas, system-connected detection with H2‑compensated CO sensors reduces false alarms compared with typical residential units.
- Place your CO detector close to an air vent or ceiling fan where air is moving too quickly for gas detection; avoid bathrooms and directly over fuel-burning appliances to limit nuisance alarms and follow height guidance in the manual and CPSC tips.
What’s Next?
To choose a reliable, cost‑effective system, start with certification (UL 2034 for residential alarms; UL 2075 for system-connected) and then match features to your home. Typical product tiers and price ranges (street prices) from current market comparisons: CO‑only plug‑in with display and battery backup ($30–$45; e.g., First Alert CO615, Kidde Nighthawk) (First Alert; Kidde Nighthawk); sealed‑battery CO‑only (often 10‑year) with optional display ($35–$60; e.g., First Alert CO710; Kidde 10‑year models) (First Alert CO710); and premium smart combination smoke+CO alarms with voice/app alerts ($110–$170; e.g., Google Nest Protect certifications; Kidde Smart with IAQ). Interconnection (wired or wireless) ensures whole‑home audibility; digital displays and peak‑level recall help diagnose incidents; voice alerts and app notifications improve clarity and response (Wirecutter; Consumer Reports). If you want cross‑platform alerts, Matter support for Smoke/CO alarms is expanding (CSA), and some ecosystems can pause HVAC during an alarm (Nest Safety Shutoff). Consumer feedback highlights the value of voice alerts, “hush” features, and app reliability, and notes nuisance alarms near garages and end‑of‑life chirps as common pain points—proper placement and timely replacement minimize these issues (CPSC safety guide; owner/editorial reviews). We’ve reviewed several smart smoke and carbon monoxide detectors and current independent guides frequently recommend premium smart combos like Google Nest Protect alongside simple UL‑listed plug‑in or 10‑year CO‑only models for value. Always verify local code requirements (e.g., 2024 IRC R315; NFPA 72 (2025)) and note that federal housing programs expect compliant alarms in assisted housing (HUD NSPIRE).
Lindsay Haskell is a home security writer for Reviews.com. Over the last few months here, she has covered medical alert systems, home security, and home automation systems, as well as home monitoring service providers.
