C O by the numbers. Fifty, thirty-five, twenty-five, nine, seventy, four hundred, two hundred — seven numbers, two scales, and mixing them up gets people hurt.

A tech drops a probe into a flue and reads eighty-seven parts per million, and shrugs. That's under the four hundred limit, right? Wrong move. Eighty-seven as-measured is not eighty-seven air-free — and a flue-gas number is not a breathing-air number. Seven C O numbers live in this trade, on two different scales. Confuse them, and someone gets hurt.

Every C O number belongs to one of two worlds. The first is breathing air — how much carbon monoxide is in the air people actually breathe, read straight off the meter in parts per million. The second is appliance flue gas — a combustion-quality score for the exhaust, expressed air-free, a corrected value. Breathing-air limits protect people. Flue-gas limits grade the burn. Never trade one for the other.

The breathing-air limits, strictest to loosest. The public health standard, E P A, is nine parts per million over eight hours, and thirty-five over one hour. Industrial hygiene, A C G I H, twenty-five over eight hours. The recommended limit, NIOSH, thirty-five over eight hours, with a ceiling of two hundred that must never be exceeded. The legally enforceable limit, OSHA, fifty over eight hours. And immediately dangerous to life, twelve hundred parts per million. These assume healthy adults, which is why the public limit is far lower.

In the field, a simpler set of action levels guides you. Nine to thirty-five parts per million: keep working, but advise the occupant and investigate the source. Thirty-six to sixty-nine: ventilate — open windows and doors, and shut down suspect appliances. Seventy or more: stop, evacuate everyone, get out of the building, and call nine one one from outside.

So what's normal? Inside a home, zero parts per million. A candle or a smoker might push it to a few parts per million, and that's tolerable. But anything above about six parts per million is unusual — treat it as a signal, not noise, and find out where it's coming from.

Now, a household C O alarm is not a low-level tool, by design. Under the residential alarm standard, U L twenty thirty-four, it must sound at seventy parts per million within one to four hours, at one hundred fifty within ten to fifty minutes, and at four hundred within four to fifteen minutes. And below thirty parts per million, it must not alarm at all for thirty days. It is built to catch emergencies, not chronic exposure.

That gap is exactly why you clip on your own low-level monitor. A personal C O monitor alarms somewhere around five to thirty-five parts per million — long before a code alarm ever stirs. It rides in your breathing zone and warns you while the number is still small. A code alarm and your monitor are not the same instrument.

Now the other scale. At the appliance, the air-free flue-gas limit is about four hundred parts per million for a furnace and two hundred for a water heater. Air-free means the dilution air has been mathematically removed. Ranges and ovens are different — they're unvented, so the code's Annex G grades them as-measured instead: about two hundred twenty-five at an oven or broiler, and just twenty-five per burner on the cooktop. The eight-hundred air-free figure you may have heard is the oven's certification limit under A N S I Z twenty-one point one, not an N F P A fifty-four threshold. Here's the trap: excess air lowers the raw reading, so a burner can look clean as-measured yet fail air-free. Correct it — multiply the measured C O by twenty point nine, over twenty point nine minus the measured oxygen percent. Our eighty-seven at five percent oxygen becomes about one hundred fourteen air-free. Same gas — a different, honest number.

Two scales, seven numbers. Breathing air is measured in parts per million and protects people — the field triple: nine, thirty-six, seventy. Flue gas is air-free and grades the burn — four hundred for a furnace, two hundred for a water heater. Correct before you compare, and carry your own low-level monitor. Next episode: the instruments.