Indoor Air Quality Monitors: What CO2 and PM2.5 Readings Mean

CO2 readings above roughly 1,000ppm generally suggest a room is poorly ventilated and stuffy rather than dangerous. PM2.5 readings climbing above the low double digits (µg/m³) indicate fine particulate matter — often bushfire smoke, cooking fumes or dust — that can irritate airways with prolonged exposure. Neither number alone diagnoses a health problem; both are prompts to ventilate, filter or investigate further.

Why Indoor Air Quality Monitoring Matters in Australian Homes

Australian homes present a particular mix of air quality challenges. Many houses, especially older ones, rely on natural ventilation rather than mechanical fresh-air systems, so CO2 can build up in closed-up bedrooms overnight. Meanwhile, bushfire season brings smoke events that can push outdoor and indoor PM2.5 to levels that make people rethink whether windows should be open at all. Add in gas cooktops, wood heaters, and increasingly airtight renovations for energy efficiency, and it's easy to see why a growing number of households are adding a dedicated air quality monitor alongside their thermostat.

A monitor doesn't fix anything by itself — it just gives you numbers. What you do with those numbers (open a window, run an exhaust fan, switch on a purifier) is where the actual air quality improvement happens.

Understanding CO2 Readings

Carbon dioxide is a reasonable proxy for how "used up" the air in a room is. As people breathe, they exhale CO2, and in a sealed room with no fresh air exchange, levels climb steadily.

What's a "normal" CO2 level

Outdoor air typically sits in the low hundreds of ppm. Indoors, with reasonable airflow, levels in the high hundreds to around 1,000ppm are common and not generally considered concerning for most healthy adults. As CO2 climbs into the 1,000–2,000ppm range, some people report feeling drowsy, and above that, stuffiness and reduced concentration are commonly reported, particularly in closed bedrooms or home offices with the door shut. These are general patterns reported in indoor air research rather than a medical diagnosis for any individual, and a monitor reading is not a substitute for professional advice if you have ongoing health concerns.

When CO2 signals a ventilation problem

If a monitor consistently shows CO2 climbing above roughly 1,500–2,000ppm overnight in a bedroom, it's usually a sign the room needs more fresh air exchange. Options include cracking a window, using a door with a gap, or running an exhaust or ceiling fan. In homes with gas heating, a sudden or unusual CO2 pattern alongside a gas appliance is worth having checked by a licensed technician, since combustion appliances should always be adequately ventilated and serviced. If you're managing a ducted gas or hydronic heating system, our guide to smart thermostat options for gas ducted and hydronic heating covers how to pair better temperature control with these older systems.

Understanding PM2.5 Readings

PM2.5 refers to fine particulate matter smaller than 2.5 micrometres — small enough to be inhaled deep into the lungs. Unlike CO2, which mostly reflects ventilation, PM2.5 reflects actual pollution sources: smoke, dust, vehicle exhaust drifting indoors, or cooking.

Bushfire smoke and PM2.5

During bushfire season, outdoor PM2.5 can spike dramatically, and that smoke finds its way indoors through gaps, open windows and door seals. A monitor that tracks PM2.5 in real time is genuinely useful here. It tells you when it's actually worth closing up the house and switching to purified, recirculated air, rather than guessing based on smell or haze outside.

Indoor sources of PM2.5

Cooking — especially frying, grilling or using a gas cooktop without adequate range hood extraction — is one of the biggest indoor PM2.5 contributors in Australian kitchens. Candles, incense, and some wood heaters also add to the load. Unlike CO2, opening a window doesn't always solve a PM2.5 problem, particularly during a smoke event, which is where filtration becomes the more useful tool.

Choosing a Monitor for Your Home

Some devices are dedicated monitors, while others combine air quality sensing with smart AC control, which can be a practical middle ground for renters and homeowners alike.

The Sensibo Elements Indoor Air Quality Monitor is built specifically for tracking PM2.5, TVOC, CO2, humidity and temperature in one device. That makes it a solid pick for households wanting a clear read on both bushfire smoke season and everyday ventilation, without buying separate sensors for each metric. The trade-off with any standalone monitor is that it only reports — it doesn't act on the readings unless paired with other smart devices.

When PM2.5 readings are consistently elevated — whether from smoke, traffic pollution or cooking — a monitor alone won't clear the air; you need filtration. The Dyson Purifier Cool Autoreact (TP07) is designed for whole-room air purification with an oscillating cooling fan. That makes it a reasonable option for QLD and WA summers, when you want purified air moving through a room rather than sitting still. It's a larger investment than a monitor alone, so it makes most sense once you've confirmed via a monitor that PM2.5 is actually a recurring issue in your home rather than assuming it.

What To Do When Readings Are High

For elevated CO2, the first response is almost always ventilation: open a window if outdoor air quality is fine, run an exhaust fan, or use a ceiling or pedestal fan to improve air circulation. Our roundup of best smart fans for Australian summer covers options that can be scheduled or automated to run when a room feels stuffy. Bathrooms and ensuites have their own humidity dynamics worth automating separately — see our guide to humidity sensors and exhaust fan automation for Australian bathrooms.

For elevated PM2.5, especially during smoke events, the better response is usually to close up the house and run a purifier on recirculate rather than opening windows and drawing more smoke in. Keep an eye on whether readings actually fall over the following hour — if they don't, check door and window seals for gaps.

Renters and Strata Considerations

Most air quality monitors and portable purifiers are plug-in devices that don't require any fixed wiring, which makes them a low-friction option for renters and apartments under strata rules. If you're considering anything that involves modifying exhaust fans, ducting or fixed electrical work, check your lease conditions or strata by-laws first. Always engage a licensed electrician for any fixed 240V wiring changes. This article doesn't constitute legal or electrical advice specific to your property.

Sources

• NSW Government: Making changes to a rental property
• Consumer Affairs Victoria: Renters making changes to the property
• NSW Government: Strata renovation rules
• Energy Safe Victoria: Electrical DDIY – Don't do it yourself

Where to buy

Retailers stocking the products in this guide. Prices and stock change often, so check the retailer before you buy. We may earn a commission from these links at no extra cost to you.

• Sensibo Elements Indoor Air Quality Monitor: Check price at eBay AU · Check price at Woolworths · Check price at James Group
• Sensibo Air PRO Smart AC Controller with Air Quality: Check price at eBay AU · Check price at Woolworths · Check price at James Group
• Dyson Purifier Cool Autoreact (TP07): Check price at Reebelo Australia · Check price at Arohi · Check price at Electronics Centre