A long-form guide to your detector, the gas it watches for, and what to actually do if you ever see a number on the screen. Written for the people who like to understand what's protecting their home.
01What it is
The detector with a display.
The AirGuard is a plug-in detector that monitors the air in your home for two of the most common indoor threats: carbon monoxide (CO) and combustible gas (natural gas, propane, methane). Unlike a standard CO alarm, it doesn't just sit silently with a green light. It shows you, in real time, exactly what's in your air.
On the front of the unit you'll find a digital screen that displays four readings continuously, all at the same time:
CO — carbon monoxide concentration in parts per million (PPM)
GAS — combustible gas as a percentage of the lower explosive limit (% LEL)
TEMP — ambient room temperature
RH — relative humidity
The first two readings are what protect your family. The other two are useful for understanding the conditions in your home — high humidity can encourage mold, low humidity can dry out airways, and temperature fluctuations near the unit can sometimes indicate a nearby HVAC issue.
Why it has a display in the first place.
Most home CO alarms are designed around a 1990s assumption: that a homeowner only needs to know two things — is there a problem (loud beep), or isn’t there (silent green light). Decades later, that thinking is still the default.
The problem is that carbon monoxide doesn’t behave like a binary. It accumulates slowly, often overnight, often at low levels that cause real harm long before any alarm threshold is reached. A green light tells you the alarm has power. It doesn’t tell you what’s actually in the air.
AirGuard exists because we believe the homeowner deserves the actual number — not a reassuring light.
02About carbon monoxide
The gas you can’t see, smell, or taste.
Carbon monoxide is produced whenever a carbon-based fuel — gasoline, natural gas, propane, kerosene, wood, charcoal — burns without enough oxygen. Cars, furnaces, water heaters, gas stoves, fireplaces, generators, and grills all produce it. In well-functioning equipment, that CO is safely vented outside. In faulty equipment, it ends up in your home.
It has no color, no scent, no taste. It doesn’t sting your eyes or make you cough. By the time most people realize something is wrong, they’ve already been breathing it for hours.
Carbon monoxide bonds to the hemoglobin in your blood about 200 times more readily than oxygen.
That bond is what makes CO so dangerous. Hemoglobin is the protein in your red blood cells responsible for carrying oxygen from your lungs to every organ in your body. When CO is in the air, your blood preferentially binds to the CO instead — your lungs are working, your heart is pumping, but your tissues are being starved of oxygen.
The result depends on concentration and exposure time. At low levels, you feel tired, headachy, foggy. At higher levels, you become disoriented, confused, and eventually unconscious. The brain and heart are most vulnerable because they need the most oxygen to function.
Why it’s worse for some people.
The same level of CO does not affect every person equally. Three groups are at higher risk:
Children — especially under 5. They breathe roughly twice as fast as adults and have smaller lungs and circulatory systems. Their blood saturates with CO faster relative to body weight.
Older adults — particularly those over 65. Reduced lung capacity, slower respiration recovery, and pre-existing cardiovascular conditions all compound the effects.
People with heart or lung conditions — anyone with COPD, asthma, anemia, or coronary artery disease will reach symptomatic levels faster than someone in good health.
This is why the standard 70 PPM alarm threshold — designed around healthy adults — is not protective for the people in your home who matter most.
A Brief History
Why detectors became required.
Residential CO alarms didn’t exist until the late 1980s. After a series of high-profile family fatalities — most notably the 1990 deaths of two children in Chicago — the industry began producing the first home detectors. Building codes started requiring them in new construction in the early 2000s. As of 2023, 36 US states require working CO alarms in residential buildings. The remaining 14 do not.
03How AirGuard works
What’s happening inside the unit.
Your AirGuard contains two independent sensors and a small processor that reads them both several times every second. Understanding how each one works helps explain why a digital reading is more useful than a simple alarm.
The electrochemical sensor (CO).
The carbon monoxide sensor inside AirGuard is electrochemical. It contains a small chamber with two electrodes and an electrolyte solution. When CO molecules drift into the chamber, they react chemically with one of the electrodes, producing a tiny electrical current. The strength of that current is directly proportional to the concentration of CO in the air — which is how AirGuard generates a precise PPM reading rather than a yes-or-no signal.
This is the same sensor technology used in professional-grade CO meters carried by HVAC technicians and firefighters. It’s accurate to within ±3 PPM in the 0–100 PPM range, which is the range that matters for early warning.
The catalytic sensor (combustible gas).
The second sensor watches for combustible gases — primarily natural gas (methane), propane, and LPG. It works on a different principle: a small heated bead in the sensor’s chamber burns any combustible gas that reaches it. The act of burning produces a temperature change, and that change is converted into a percentage of the lower explosive limit, or %LEL.
Why %LEL? Because for gases like methane, the actual hazard is explosion, not toxicity. The “lower explosive limit” is the minimum concentration at which a gas could ignite. AirGuard alerts well before that point.
The continuous self-test.
One of the most common failure modes of standard CO alarms is silent sensor expiration. The electrochemical reaction that makes them work degrades over time — typically 5 to 7 years — and once the sensor stops responding, the alarm becomes a green-lit decoration. The homeowner has no way to know.
AirGuard runs a small diagnostic on its sensors every few seconds. If the CO sensor stops responding, the gas sensor fails, or the unit detects an internal fault, the yellow LED lights up and the screen displays an alert. You’re not relying on a green light to tell you the truth — the unit verifies itself.
04What the numbers mean
Reading the display.
Most of the time your screen will show 0 PPM CO and 0% LEL gas. That’s the goal. Clean air. Nothing to worry about.
Occasionally you might see a small reading appear briefly — 1, 2, sometimes 4 PPM — after grilling indoors, lighting a fireplace, or running a gas stove for a long cooking session. These small spikes are normal and will return to 0 within a few minutes as the air clears.
What matters is when the number doesn’t return to 0, or when it climbs steadily. The table below explains what’s happening at each level.
0–9PPM
Normal background. Outdoor air typically reads 0–2 PPM. Brief spikes during cooking or after starting a fireplace are common and harmless.
10PPM
Early warning range. Not yet harmful, but high enough that something is producing CO. Open windows, ventilate the area, and investigate the source. Most issues are caught and fixed at this stage.
30PPM
Headaches and fatigue begin. Symptoms are easily mistaken for the flu or a long workday. Standard UL 2034 alarms remain silent.
50PPM
Cognitive effects. Nausea, vomiting, difficulty concentrating. Children may seem unusually drowsy. Standard alarms still silent.
70PPM
Standard alarms allowed to sound. UL 2034 permits up to 4 hours of sustained 70 PPM exposure before the alarm is required to activate.
150+PPM
Loss of consciousness possible. Severe disorientation. Without immediate evacuation, fatal exposure can occur within an hour.
The indicator lights.
Beneath the screen are three LED indicators that give you status at a glance:
Green — Power is on and the unit is monitoring. This is the normal state.
Yellow — Sensor is warming up (only on first power-on for ~30 seconds), or there is a fault. If yellow stays solid for more than 5 minutes, contact support.
Red — Alarm. Flashes with the audible buzzer when CO or gas is detected at dangerous levels.
05Where leaks come from
Sources of CO in a typical home.
If your AirGuard ever shows a reading above 0 that doesn’t clear within 10–15 minutes, there’s a source somewhere. Knowing the most common ones can help you find it quickly.
The furnace and water heater.
The single most common source of residential CO is a cracked heat exchanger inside a forced-air furnace. The exchanger is the metal partition that separates combustion gases from the air being pushed through your vents. When it cracks — usually from age or thermal stress — combustion byproducts including CO leak directly into the supply air going to every room.
Gas water heaters can fail similarly. A blocked or disconnected flue pipe, corroded venting, or improper installation can cause exhaust to spill back into the utility room and from there into the rest of the house.
The kitchen.
Gas stoves emit small amounts of CO during normal operation. In a well-ventilated kitchen with a working range hood, this is rarely a problem. In a closed kitchen without ventilation — particularly when the oven is running for long periods — levels can climb. A blocked or improperly adjusted burner can produce significantly more CO than a properly tuned one.
The garage.
Even a brief two-minute warm-up of a car in an attached garage can release enough CO to saturate the garage and seep into the home through gaps under the door, shared walls, and HVAC returns. If your garage is attached, never start a vehicle inside. Open the garage door, then start the engine.
Generators, grills, and other portable sources.
Backup generators kill more people from CO each year than any other single source. They produce enormous quantities of CO and must always be operated outside, at least 20 feet from any window or door, and never in a garage — even with the door open.
Charcoal grills, propane grills, hibachis, and any other portable combustion device should never be operated indoors or in an enclosed porch. A grill that has finished cooking but is still warm continues to off-gas for hours.
The fireplace and chimney.
A blocked chimney — by a bird’s nest, leaves, soot buildup, or a closed damper — can cause smoke and CO to back-draft into the living room. Annual chimney inspection is recommended for any home with a wood-burning or gas fireplace.
The pattern across nearly every CO incident is the same: the source had been failing for weeks or months before anyone noticed.
06If the alarm sounds
What to do if you ever see a number.
The moment AirGuard’s red LED flashes and the buzzer sounds, you have time — but not unlimited time. Here’s the right order of actions.
Step 1 — Get everyone out.
Don’t stop to investigate. Don’t go looking for the source. Get every person and pet out of the building and into fresh air immediately. Open the front door on your way out if it’s on your path, but don’t add steps. You can investigate later. Lives first.
Step 2 — Call 911 from outside.
Once you’re outside, count heads. Confirm everyone is out. Then call 911. Tell them you have a confirmed CO alarm with a digital reading — they’ll dispatch the fire department with proper equipment.
If anyone has symptoms — headache, dizziness, nausea, confusion, drowsiness — tell the dispatcher. CO poisoning is treated with high-flow oxygen, and the sooner that begins, the better the outcome.
Step 3 — Don’t re-enter until you’re told it’s safe.
Wait for the fire department. They’ll bring a calibrated CO meter, ventilate the home, and identify the source. Do not re-enter to grab valuables, check on a pet, or turn off appliances. Even brief re-entry at high CO levels can cause loss of consciousness.
Step 4 — Find the source before re-occupying.
Before you and your family return to sleeping in the home, the source needs to be identified and fixed. This usually means an HVAC technician inspecting the furnace, water heater, and gas appliances. Do not go back to a home where the source is unknown — even if AirGuard now reads 0. The conditions that caused the leak may recur the next time the appliance cycles on.
When the reading is low
Below 10 PPM, but not zero.
If your AirGuard shows a small reading (1–9 PPM) that persists or returns repeatedly, you’re not in immediate danger but something is producing CO. Common culprits: a gas stove that needs adjustment, a fireplace with a partially blocked chimney, an attached garage with insufficient sealing, or an aging water heater. Have an HVAC tech inspect your appliances. The earlier you find a small problem, the cheaper and safer the fix.
07Caring for your detector
Keeping AirGuard healthy.
AirGuard is designed to be effectively maintenance-free, but a few small habits will keep it working well for years.
Once a month — press the test button.
The test button verifies the buzzer, the LED, and the alarm circuitry. It takes 5 seconds. Make it part of the same monthly habit as changing the smoke alarm batteries — even though AirGuard doesn’t have batteries.
Every few months — wipe it clean.
Dust can accumulate over the sensor vents on the front and side of the unit. Once every few months, wipe the unit gently with a dry microfiber cloth. Don’t use cleaning sprays, water, or solvents — moisture can damage the sensor.
Watch the yellow LED.
If the yellow indicator turns on and stays on for more than a few minutes (outside of the initial first-plug warm-up), it means AirGuard has detected an internal issue. Email support@primo-goods.com with your order number and we’ll send a replacement under your lifetime warranty.
About the sensor lifespan.
The electrochemical CO sensor inside AirGuard has a typical lifespan of 5–7 years. Unlike standard alarms that fail silently, AirGuard will display an on-screen alert when the sensor approaches end-of-life. When that happens, contact us — your unit is covered under our lifetime replacement warranty.
Where to plug it in
The right height.
Mount AirGuard at breathing height — about 5 feet (1.5 m) off the ground — in any room where your family spends significant time. Hallways near bedrooms, the kitchen, and rooms adjoining gas appliances are ideal. Avoid bathrooms, laundry rooms, garages, and outdoor outlets. Carbon monoxide is roughly the same density as air, so it spreads evenly through the room — there’s no benefit to putting the detector on the ceiling or floor.
A Final Note
Thanks for taking the time to understand it.
The single most valuable thing your AirGuard does is something you’ll hopefully never need: alert you early. We hope your screen reads 0 every day for years. But on the rare day it doesn’t, you’ll know exactly what to do — because you read this.
Questions about anything in this guide? Our team answers personally — usually within 4 hours, weekends included.
