Carbon Monoxide vs. Gas Leak: The Chicago Winter Confusion That Sends People to the ER
Every Chicago winter, two completely different dangers get mixed up in the same panicked phone calls: a natural gas leak and carbon monoxide. One announces itself with a smell; the other is invisible and odorless. One means “get out and call the utility”; the other means “get out and get medical attention.” Confusing them wastes minutes you may not have — so here’s how to tell them apart in seconds, and exactly what to do for each.
Gas Leak vs. Carbon Monoxide: The Side-by-Side
| Natural gas leak | Carbon monoxide (CO) | |
|---|---|---|
| What it is | Unburned fuel escaping from piping, a fitting, or an appliance | A poison gas produced by incomplete combustion in a furnace, boiler, water heater, range, or car |
| Smell | Rotten eggs / sulfur (an additive called mercaptan — raw natural gas is odorless) | None. Completely odorless and invisible |
| How you detect it | Your nose, hissing sounds, dead vegetation outdoors, high bills — plus professional instruments | Only a CO alarm. Your senses cannot detect it |
| Body symptoms | Usually none at typical leak levels — the smell is the warning | Headache, dizziness, nausea, fatigue, confusion — often improving when you go outside |
| Main risk | Fire and explosion | Poisoning — it displaces oxygen in your blood |
| First response | Evacuate, no switches or flames, call 911 + Peoples Gas from outside | Evacuate, call 911, tell them “possible CO,” seek medical evaluation |
| Who fixes the cause | Utility up to the meter (free); licensed gas fitter past it | HVAC/combustion service for the appliance — and a gas fitter if venting or supply piping is implicated |
Why Chicago Winters Manufacture This Confusion
The two problems spike together for the same seasonal reasons. From November through March, furnaces and boilers run near-constantly — more combustion means more opportunities for a cracked heat exchanger, a blocked flue, or a backdrafting water heater to produce CO. At the same time, cold contraction works on the city’s aging threaded gas joints, so actual gas leaks peak in the deep-freeze weeks too (we wrote a whole guide on why leaks spike in Chicago winters). And because every window is sealed shut, neither problem dilutes the way it would in July — smells concentrate, CO accumulates, and symptoms show up faster. Same season, same appliances, completely different emergencies.
The Furnace Can Cause Both — Here’s the Difference
A furnace sits at the intersection of these two risks, which is exactly why people conflate them. On the fuel side, the gas piping, shutoff valve, and union feeding the furnace can leak unburned gas — that’s a gas leak, it smells, and it’s repair territory for a licensed gas fitter (typical fitting repairs run $120–$300). On the combustion side, a cracked heat exchanger or blocked venting lets exhaust — including CO — escape into your living space instead of up the flue; that’s odorless, only an alarm catches it, and the fix is furnace service or replacement. One appliance, two failure modes, two different professionals. If a technician condemns a furnace for a cracked exchanger and you also want the supply piping verified before the new unit goes in, that’s a pressure test ($75–$150) — the ideal moment for one, honestly, since the line’s already being worked on.
Detector Rules Chicago Homes Actually Live Under
- ✓ CO alarms are required by law in Illinois homes within 15 feet of every sleeping room when the home has fossil-fuel appliances or an attached garage. If your rental doesn’t have one, that’s a landlord obligation — our who-pays guide covers how to escalate.
- ✓ Test monthly, replace per the unit’s lifespan — CO sensors expire (typically 5–10 years); the date is printed on the back. An expired alarm is decor, not protection.
- ✓ A “natural gas detector” is a separate device — optional but genuinely useful in homes where someone’s sense of smell is diminished, and worth considering for basements with aging piping.
- ✓ Alarms don’t fix causes. A CO alarm that keeps chirping “for no reason” or a recurring faint gas smell both mean the same thing: stop resetting and start diagnosing.
Questions Chicagoans Actually Ask
My CO alarm went off once, then stopped. Can I ignore it?
No — CO levels fluctuate with appliance cycles and drafts, so “it stopped” often means “it dipped below the alarm threshold,” not “it’s gone.” Ventilate, get everyone out if anyone has symptoms, call 911 if in doubt, and have the combustion appliances inspected before the next cold snap forces them to run harder.
I smell something odd but it’s not quite rotten eggs. Which problem is it?
Possibly neither — musty sewer gas, a dead animal in a wall, or an overheating motor all get reported as “gas.” But mercaptan’s smell does vary person to person, so treat any sulfurous or unexplained persistent odor as a gas leak until proven otherwise: leave and call it in. The utility check is free, and instrument-based leak detection ($75–$400) settles ambiguous cases definitively.
Can a gas leak set off my CO alarm?
Generally no — CO alarms sense carbon monoxide specifically, not methane. That’s precisely why the two-detector distinction matters: your nose (plus optionally a gas detector) covers leaks; only a CO alarm covers CO. Combo units that sense both exist and are a reasonable upgrade.
Headaches every evening at home that fade at work — paranoid to suspect CO?
Not paranoid — that pattern (symptoms tied to a location, improving away from it) is textbook low-level CO exposure and worth taking seriously. Check your alarm’s age, get a working one tonight if needed, and book a combustion inspection of the furnace and water heater. Mention the pattern to a doctor too; low-level exposure is testable.
Whose job is the fix — gas company, HVAC, or gas fitter?
By cause: the utility handles everything up to your meter free (1-866-556-6002); a licensed gas fitter handles leaking piping, valves, and connectors past the meter; HVAC/combustion techs handle heat exchangers, burners, and venting that produce CO. Describe the symptom when you call and any honest pro will tell you if you’ve reached the wrong trade.
Is one of these more dangerous than the other?
They’re dangerous differently: gas leaks carry explosion risk that scales with concentration; CO quietly injures at levels you can’t perceive. Statistically CO sends far more people to the ER each winter precisely because it gives no warning. Respect both; alarm for one, nose for the other.
Smell Gas? Don’t Diagnose It From the Couch.
Same-day gas leak detection and repair across Chicago — instruments, not guesswork, with the pressure-test paperwork that proves the fix. CO concerns? I’ll tell you straight when it’s an HVAC job instead.
☎ Call (773) 200-2148Related guides: Why Gas Leaks Spike in Chicago Winters · Who Pays for a Gas Leak in a Rental? · All guides