Why CO coverage matters in old rentals

Old rentals carry a particular kind of risk: the systems that make them livable are often the same systems most likely to leak carbon monoxide. Aging furnaces, drafty flues, poorly vented water heaters, patched-together gas lines, even a garage below a unit—none of this looks dramatic from the sidewalk. That is exactly the problem. Carbon monoxide, or CO, gives off no smell, no color, no warning cough. People don’t “notice it in time.” They get headaches, feel oddly tired, blame stress, and go back to bed.

Why older rentals are more vulnerable

CO is produced when fuel does not burn completely. In a newer building, sealed combustion appliances, modern venting, and stricter code enforcement reduce the odds of buildup. In older rentals, failure points multiply.

A few common ones show up again and again:

• Cracked heat exchangers in old furnaces
• Rusted or disconnected vent pipes
• Gas stoves used for supplemental heat
• Chimneys with weak draft or blockages
• Portable generators or charcoal grills used too close to windows
• Attached garages that allow exhaust to migrate indoors

The Centers for Disease Control and Prevention has long treated CO poisoning as a major public health issue, with more than 400 accidental deaths and over 100,000 emergency department visits in the U.S. each year. Those figures exclude fires. In practice, the burden falls hardest on people in housing they do not fully control—renters, especially in older stock.

The landlord detector may exist, but that doesn’t mean coverage is adequate

This is where “coverage” matters more than simply “having a detector.” One aging CO alarm in a hallway is not the same as meaningful protection.

Proper CO coverage means the alarm can detect gas where exposure is likely to happen before occupants are significantly impaired. In older rentals, that usually requires attention to placement:

• Outside each sleeping area
• On every habitable level
• Near fuel-burning appliances, but not so close that nuisance alarms become constant
• In basement or utility-adjacent spaces when allowed by manufacturer guidance

A 1920s duplex with a basement furnace and bedrooms upstairs is a very different risk profile from a studio with electric heat. Yet many older rentals still rely on a single outdated unit, sometimes installed years past its replacement date. Most CO alarms have a limited service life, often 5 to 10 years. After that, the sensor chemistry degrades. The plastic box may still sit there looking official, quietly useless.

Why symptoms get missed in rentals

CO poisoning often mimics ordinary life badly. That sounds strange, but it is true. Mild exposure can feel like flu, fatigue, dizziness, nausea, or a pounding headache after a long day. In winter, tenants may assume the dry heated air is making them feel lousy. In a crowded building, several residents getting headaches at once might be blamed on a virus.

That ambiguity is dangerous. At higher concentrations, judgment and coordination deteriorate. The person most in need of acting fast may no longer think clearly enough to do it.

CO doesn’t need a dramatic leak to be harmful. Low-level exposure over hours can still create a medical emergency.

Old rentals create one more complication: shared infrastructure

A tenant may maintain their unit perfectly and still face CO from equipment they never touch. A neighboring boiler room, a backdrafting water heater in another apartment, vehicle exhaust from an attached garage, a blocked chimney serving multiple units—shared systems make responsibility blurry and exposure pathways messy.

That is why robust CO coverage is not just a gadget question. It is a building-risk question.

What renters should check

• Manufacture date on every existing CO alarm
• Whether alarms are placed near sleeping areas
• Whether the unit has gas heat, gas cooking, a fireplace, or garage adjacency
• Whether anyone in the home has recurring unexplained headaches or nausea indoors
• Whether vented appliances show soot, rust, or scorch marks

If the detector is older than the manufacturer’s service life, it should be replaced. Not “kept an eye on.” Replaced.

The practical takeaway

In old rentals, carbon monoxide risk is less about panic and more about probability. Older combustion equipment fails more often. Venting systems age. Maintenance can be delayed. Tenants usually cannot inspect a heat exchanger or climb onto a roof to check a flue cap. CO coverage becomes the last clean line of defense when everything upstream is uncertain.

And that small plastic alarm on the wall? In a prewar building with a moody furnace, it may be the only thing in the apartment that tells the truth.