The simplest effective approach is a mix of methods matched to risk: run a consumer-grade monitor for two or more weeks to catch patterns in particles, VOCs, and CO2, deploy a long-term radon detector for at least three months before making any decisions, treat carbon monoxide as an emergency pollutant requiring continuous alarms, and call a certified professional when results are ambiguous, symptoms persist, or the situation carries legal weight. Begin monitoring now, log your daily activities alongside the readings, and plan a follow-up test or a professional visit if the numbers do not settle into a clear pattern.
TL;DR:
- Using consumer monitors for two weeks and long-term radon tests lasting three months helps identify patterns and risks related to air quality.
- Prioritize testing for radon, because it is common and slow-acting, and ensure carbon monoxide alarms are functioning to manage immediate danger.
- Avoid relying solely on low-cost sensors for accurate chemical identification; lab analysis is necessary for specific VOCs and detailed results.
- Place monitors in the breathing zone, away from sources, and log activities and symptoms to better interpret readings and identify sources.
- Call professionals for visible mould, unexplained high results, or when needing legally defensible documentation to guide effective remediation.
Table of Contents
- Which pollutants matter most and how to prioritize testing
- Testing methods explained: matching the tool to the question
- How to run tests correctly: placement, duration, and logging
- Interpreting results: benchmarks, spikes, and averages
- Radon and carbon monoxide: the two exceptions that need special handling
- DIY monitoring versus hiring a professional
- What to do after testing: mitigation and proving it worked
- Testing tells you one thing at a time
- Getting professional testing and the right equipment
- Sources
- FAQ
Which pollutants matter most and how to prioritize testing
Not every pollutant deserves the same urgency. Carbon monoxide can kill within hours, radon causes cancer only after years of exposure, and elevated CO2 mostly signals poor ventilation rather than direct toxicity. Sorting pollutants by how quickly and severely they can hurt you tells you where to spend your first testing dollars.
Fine and coarse particles (PM2.5 and PM10) come from cooking, candles, and outdoor infiltration and are linked to respiratory and cardiovascular strain over time. Volatile organic compounds, often summarized as TVOCs, come off paint, furniture, cleaning products, and new flooring, and some individual compounds like formaldehyde carry their own chronic risk profile. Carbon monoxide is acute and can be fatal within a single exposure, which is why it gets separate handling later in this guide. Carbon dioxide is not directly toxic at typical indoor levels but works as a reliable proxy for how well a space is ventilated. Radon is a slow-acting carcinogen with no smell or color, detectable only through dedicated testing. Moisture and mould indicators, meanwhile, point to a structural problem more than a single measurable gas.
A practical order of operations looks like this:
- Life-safety first: confirm a working CO alarm and smoke detector before anything else.
- Chronic risks next: test radon because it is common, invisible, and cumulative.
- Ventilation check: track CO2 as an indicator of how much fresh air a room actually gets.
- Particle and VOC screening: run these when allergy symptoms, new furnishings, or renovation work suggest a source.
- Mould investigation last, and only when there is visible growth, moisture damage, or a musty odor, since air sampling for mould without a physical inspection rarely adds useful information.
That order keeps you from spending money on a VOC lab panel while a cracked furnace heat exchanger sits unnoticed in the basement.
Testing methods explained: matching the tool to the question
Every pollutant has more than one way to measure it, and the right choice depends on whether you need a quick screen, a defensible number, or a legal-grade report.
Consumer and low-cost monitors use optical sensors for particles, non-dispersive infrared (NDIR) sensors for CO2, and electrochemical or metal-oxide sensors for VOCs and CO. They are inexpensive, give continuous readouts, and are genuinely useful for spotting patterns such as a spike every time someone cooks or a slow CO2 climb overnight. The EPA’s guidance on low-cost monitors notes that these devices are generally available at consumer prices and should run for at least two weeks in the main living areas to capture a representative pattern, though they do not give a complete picture of air quality on their own.
For more precise or legally defensible numbers, other methods take over:
- Passive badges and sorbent tubes absorb VOCs over days to weeks and are sent to a lab for gas chromatography analysis, giving speciated results rather than a single TVOC number.
- Alpha-track and charcoal canister detectors measure radon passively over different timeframes, with alpha-track suited to long-term testing and charcoal to short screens.
- Active pumped sampling and canisters pull a known volume of air through a filter or into a container, letting a lab identify and quantify specific chemicals, useful when you need to know exactly which VOC is present.
- Gravimetric filter sampling weighs a filter before and after a set sampling period to get an accurate PM mass concentration, the reference method against which optical sensors are checked.
- Microbial (mould) air sampling collects spores onto a growth plate or cassette, but Health Canada’s guidance for indoor air quality professionals cautions that routine airborne mould sampling is not usually recommended because results are hard to interpret without a visual inspection first.
Direct-reading instruments are enough when you only need to know whether a level is roughly high or low, such as confirming a CO alarm’s reading. Lab analysis becomes necessary when you need to identify a specific compound, document a result for a real estate transaction or insurance claim, or confirm a borderline finding from a cheaper sensor. A PM and gas measurement review summarizes the trade-off well: optical counters are convenient and continuous, gravimetric sampling is slower but far more accurate for mass concentration.
How to run tests correctly: placement, duration, and logging
A well-placed monitor for two weeks tells you more than an expensive device placed badly for two days. The goal is a representative sample of the air you actually breathe, not the air near a window or a heating vent.
- Place the monitor in the breathing zone, roughly 3 to 6 feet off the floor, in a spot where people spend real time, following the placement guidance in Health Canada’s residential air quality guidelines.
- Keep it away from direct sources such as stovetops, fireplaces, and open windows, and away from drafts that skew short-term readings.
- Run consumer monitors for at least two weeks to capture normal weekly patterns, including weekends and any recurring activities like cooking or cleaning.
- For radon, commit to a long-term test of several months, ideally during a season when a home is closed up and results are most representative, per Health Canada’s radon measurement guide.
- Allow new sensors a short stabilization period before trusting the first readings, and follow the manufacturer’s calibration and maintenance schedule.
- Keep a simple log of dates, activities, and anything unusual (a new candle, painting, guests smoking) so spikes can be traced to a cause rather than left as a mystery.
If you are trying to figure out how much outdoor air is getting in, place a second monitor outside or near an entry point for comparison. If a complaint is localized to one room, such as a bedroom with recurring headaches, run a second device there rather than relying on a single reading from the living room. A short homeowner air quality assessment survey can help structure this kind of walk-through before you even turn a monitor on.
Pro Tip: Run your monitor’s first 48 hours as a throwaway period. Sensors often read erratically while they settle, and treating early data as final leads to false alarms or false reassurance.
Interpreting results: benchmarks, spikes, and averages
A number on a screen means nothing until you compare it to a benchmark and understand whether you are looking at a spike or a pattern.
Health Canada’s residential air quality guidelines set a long-term CO2 exposure limit of 1,000 ppm as a 24-hour average, a useful proxy for whether a space gets enough fresh air rather than a toxicity threshold in itself. The same guidance sets a 1-hour short-term limit for carbon monoxide at levels considered harmful in short exposures, a figure that matters because CO is directly toxic at far lower concentrations than CO2.
- CO2 above 1,000 ppm as a sustained average points to a ventilation problem, not an emergency, and calls for improved air exchange rather than evacuation.
- Any CO alarm trip or reading approaching the short-term limit is an immediate-response situation: ventilate and evacuate first, investigate the source second.
- PM and VOC spikes tied to a specific activity (cooking, cleaning, a new furniture delivery) are usually self-limiting and best addressed by source control or ventilation during that activity.
- A slow upward trend over weeks, rather than isolated spikes, suggests a persistent source worth investigating rather than a one-time event.
When documenting results for yourself or a professional, record the average, the peak, the time of the peak, and what was happening at that time. A single high reading without context is far less useful than a week of data showing when and why levels rose.
Low-cost sensors also have known accuracy limits, particularly for VOCs and PM, where cheaper electrochemical and optical sensors can drift or misread specific compounds. The EPA’s guidance on low-cost monitors recommends treating these devices as screening tools rather than final answers, and reserving lab-confirmed sampling for situations where an exact concentration or a specific chemical identity actually changes the decision you make.
Radon and carbon monoxide: the two exceptions that need special handling
Radon and carbon monoxide do not fit the general “monitor for patterns” approach because one is a slow, invisible carcinogen and the other can be fatal in hours.
Radon testing works best with long-term devices. Health Canada’s radon measurement guidance recommends a minimum of three months of testing using a C-NRPP-approved device before making any mitigation decision, with heating season preferred because homes are more closed up and results better reflect worst-case exposure. A short-term test can flag a potential problem, but any elevated short-term result should be confirmed with a long-term test rather than acted on immediately.
- Alpha-track detectors are inexpensive, passive, and well suited to the three-month-or-longer test that decisions should be based on.
- Electret ion chamber devices offer similar long-term accuracy and are common in professional deployments.
- Charcoal canisters work for quick 2 to 7 day screens but are not a substitute for a long-term result.
- Continuous radon monitors give real-time data and trend graphs, useful for tracking whether a mitigation system is working.
Carbon monoxide is different again: it is a life-safety issue, not a monitoring project. Every home needs a continuously operating CO alarm, and any alarm trip means evacuate first, ventilate second, and identify the combustion source (furnace, water heater, fireplace, attached garage) before anyone re-enters. If results confirm radon above the action level Health Canada references, or if a furnace inspection turns up a cracked heat exchanger, the next step is a mitigation referral or HVAC repair, not another round of DIY testing.
DIY monitoring versus hiring a professional
DIY testing covers a lot of ground: screening for obvious problems, tracking day-to-day patterns, and catching easy fixes like a dirty filter or a blocked vent. It falls short when the mould is hidden behind a wall, when a result needs to hold up in a real estate transaction or insurance claim, when VOC sources are numerous and need lab speciation to pin down, or when a radon result sits above the action level and a mitigation system needs to be designed and verified.
Professionals add three things a consumer monitor cannot: a systematic inspection of moisture, ventilation, and source pathways before any sample is taken; hypothesis-driven sampling aimed at answering a specific question rather than testing everything; and formal reports with documented chain of custody when results need to stand up to scrutiny. Health Canada’s guidance for professionals is explicit that sampling without an inspection first can be misleading, which is exactly the gap a certified inspector fills.
Certified testing and mitigation services are available for radon, mould, and asbestos, emphasizing personal, responsive service to provide clear answers during stressful situations. Our DIY versus professional inspection guide breaks down which scenarios call for which approach in more detail.
Pro Tip: Before a professional visit, gather your monitor logs, dated photos of any staining or damage, and a short written history of symptoms. It turns a first appointment from a blind inspection into a targeted one.
What to do after testing: mitigation and proving it worked
Results only matter if they lead to action, and the action almost always follows the same order: control the source first, ventilate second, filter third.
- Source control means fixing the leak, removing the mouldy material, sealing the radon entry point, or repairing the combustion appliance rather than just masking the problem.
- Ventilation through open windows, exhaust fans, or a mechanical ventilation system addresses CO2 buildup and diluted VOCs quickly and cheaply.
- Filtration, using HEPA units or a properly MERV-rated furnace filter, catches particles that source control and ventilation cannot eliminate on their own; our guide to improving indoor air quality covers filter selection in more detail.
For sudden spikes, open windows, run exhaust fans, and stop the activity causing the spike before considering it solved. Escalate to a professional when the fix requires a radon mitigation system, asbestos removal, or mould remediation beyond a small, contained area.
Prove the fix worked by repeating the same test, in the same spot, for the same duration as your baseline, and keep a simple record of dates, device types, and locations for both rounds.
Testing tells you one thing at a time
The biggest mistake I see is people buying an all-in-one air quality monitor expecting it to answer every question at once. It cannot. Define the question first: are you worried about ventilation, a specific smell, a health symptom, or radon, and then pick the method built for that question.
If you do nothing else, start a two-week pattern log with a consumer monitor, get a long-term radon test running, and keep your CO alarm current. Everything more complicated, hidden mould, legal documentation, a radon result above the action level, belongs with a certified professional.
— Mark
Getting professional testing and the right equipment
If your own testing turns up a result you cannot explain, or you would rather skip the two-week screening period and go straight to a certified answer, Simon Air Quality handles the pollutants covered in this guide directly. Our services page lists radon measurement and mitigation, mould testing and remediation, asbestos testing and removal, and VOC and emissions testing, each staffed by certified technicians.
A typical engagement runs from inspection to targeted sampling to lab analysis to a written report with remediation recommendations, so you leave with a documented result rather than a single number with no context. If you would rather test on your own terms first, our product catalog includes long-term radon test kits, continuous radon monitors, and air purifiers suited to the filtration step described above.
- Radon: book a radon measurement or buy a long-term radon test kit to run yourself.
- Mould: schedule mould testing when visible growth or musty odors show up, with remediation and clearance testing available if results confirm a problem.
- Asbestos: request asbestos testing before any renovation that disturbs older insulation, plaster, or flooring.
Reach out through the services page to book an assessment, or browse the product catalog if a monitor or test kit is all you need right now.
Sources
Health Canada’s residential air quality guidelines set the CO2 and CO benchmarks used throughout this guide. Its radon measurement guide and guidance for professionals cover testing duration and inspection-first practice. The EPA’s low-cost monitor guidance explains what consumer sensors can and cannot tell you, while a peer-reviewed measurement methods review compares optical and gravimetric particle sampling in more technical depth.
- Guidance for indoor air quality professionals – Health Canada
- Low-cost air pollution monitors and indoor air quality – US EPA
FAQ
Is there a way to test indoor air quality yourself?
Yes, consumer monitors can track particles, VOCs, and CO2 continuously, and passive radon detectors let you test for radon without professional help. Run a monitor for at least two weeks to see realistic patterns, and use a long-term radon device for at least three months before acting on the result.
How do you tell if you have poor indoor air quality?
Sustained CO2 readings above the 1,000 ppm 24-hour benchmark point to weak ventilation, while recurring PM or VOC spikes tied to cooking, cleaning, or new furnishings suggest a source worth addressing. Persistent symptoms like headaches or respiratory irritation that ease when you leave the home are also a practical signal worth investigating with a monitor.
How do you know if the air in your house is toxic?
A working carbon monoxide alarm is the fastest way to catch an acute toxic threat, since any alarm trip means evacuating and ventilating immediately. For chronic risks like radon, only a long-term certified test can confirm whether levels sit above the action threshold that Health Canada recommends acting on.
Is there an app to test indoor air quality?
Apps themselves do not measure air, but many consumer monitors pair with an app to log and graph readings from their built-in optical, NDIR, or electrochemical sensors over time. Treat the app’s numbers as useful for spotting patterns rather than as a lab-grade result, since low-cost sensors have known accuracy limits for VOCs and fine particles.
