Monitoring indoor air quality in 3 locations is useful only when you know what each monitor can detect and whether the readings were collected in comparable conditions. The available information does not identify the locations, devices, pollutants, dates, or readings, so it cannot support claims about what happened at those sites. The nine lessons below explain how to plan and interpret a three-location comparison without presenting general guidance as project findings.
1. Define what “indoor air quality” means for your comparison
An indoor air quality monitor does not measure air quality as a single, all-purpose score. It detects only the pollutants or environmental factors covered by its installed sensors. Depending on the device, these may include particulate matter, radon, carbon monoxide, carbon dioxide, formaldehyde, volatile organic compounds (VOCs), temperature, or humidity. A monitor may contain one sensor or several.
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The U.S. Environmental Protection Agency (EPA) summarizes the role of these devices this way: “Low-cost air pollution monitors can provide some information about the levels of specific pollutants or environmental factors in a space and how those pollutant levels or environmental factors are impacted by activities in a space.” EPA’s guide to low-cost monitors and indoor air quality also cautions that a monitor’s readings do not describe every aspect of indoor air.
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Before comparing locations, list the pollutant or condition you want to assess and confirm that each device actually measures it. If one site has a particulate-matter sensor and another does not, their particulate readings cannot be compared.
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2. Plan the comparison before collecting data
A useful multi-location study starts with a clear question and a consistent plan. Decide in advance which pollutants or conditions matter, what locations and time periods to sample, how sensors will be placed, and how you will record activities or events that could affect readings. Note how you will handle missing data and whether readings will be raw or adjusted.
EPA’s 2022 Enhanced Air Sensor Guidebook covers planning, sensor selection and setup, collection, evaluation, analysis, interpretation, communication, and action. EPA also describes advance planning as important to quality assurance and useful data in its guidance on air sensor use and study design.
3. Make the three locations comparable—or explain why they are not
Using the same monitor and procedure at each location makes a comparison easier to interpret, but it does not make different rooms or time periods identical. Record the device and sensor types, placement, dates and duration, relevant activities, and any data gaps for every location. If devices or procedures differ, identify those differences before presenting the readings.
A comparison should state its axes explicitly: what pollutant or condition was measured, which sensor and device were used, where and when readings were taken, what was happening at the time, and whether the numbers are raw or adjusted. Without those details, an apparent difference may reflect the instrument or sampling conditions rather than a meaningful difference between locations.
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4. Place monitors where people breathe
For general indoor monitoring, EPA recommends following the manufacturer’s placement instructions and positioning the sensor in the typical breathing zone, approximately 3–6 feet above the floor. Keep airflow around it unobstructed and avoid placing it close to a pollution source or an air cleaner, where a local plume or cleaned-air stream could skew the reading. See EPA’s guide to siting and installing air sensors for the full guidance.
This is a general placement recommendation, not a guarantee that every reading represents an entire room. Record the monitor’s location and height so readers can interpret the result in context.
5. Treat windows, doors, and vents as potential sources of variation
A monitor near a window, door, or HVAC duct may experience rapidly changing temperature, humidity, or pollutant conditions. Those readings can be less representative of average indoor concentrations than readings taken in a more typical occupied area. Note nearby openings and vents, and record relevant changes—such as a window being opened—when they occur.
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6. Interpret changes alongside activities and conditions
Readings can shift with room activities and with the conditions around the sensor. EPA notes that placement, time in use, data-processing methods, temperature, relative humidity, and multiple contaminants can affect results. A reading that rises during an activity may be a useful clue about timing, but a low-cost monitor alone may not establish the source or identify every substance involved.
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Keep an activity log alongside the measurements. Record relevant events and conditions rather than treating a plotted line as self-explanatory.
7. Do not mistake a display or alert for a health standard
EPA says there are no widely accepted indoor concentration limits for most pollutants, and that alert thresholds on consumer monitors are set by manufacturers. A device’s color indicator or notification should therefore be read according to its documentation, not assumed to be a universal safety threshold. Price alone does not establish how well a monitor performs.
EPA also says information on the accuracy and precision of marketplace monitors is limited. Use such monitors for informational, non-regulatory monitoring and for observing trends; do not present their readings as regulatory measurements or as a diagnosis of a health problem.
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EPA recommends keeping indoor relative humidity between 30 and 50 percent and suggests checking it with a moisture or humidity gauge. That range is general indoor-air guidance—not a result from any particular three-location comparison. If humidity is one of the conditions being compared, confirm that each monitor measures it and record the readings and circumstances consistently.
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A monitor that does not include a humidity sensor cannot provide those measurements. Likewise, a humidity reading says nothing by itself about pollutants the device does not detect.
9. Account for sensor aging and limits when reporting results
Air monitors have limited lifespans and may become less reliable over time. Check the manufacturer’s instructions for service, calibration, or replacement guidance, and record the device’s age or relevant maintenance if known. EPA notes that monitor results can also be influenced by processing methods and the presence of multiple contaminants, so explain how the data were handled and avoid overstating precision.
When publishing a three-location comparison, distinguish observations from conclusions. State what the sensors recorded, what contextual information is available, and what the devices could not establish. EPA’s indoor air quality guide also discusses radon testing; do not assume a general-purpose monitor detects radon unless its specifications confirm that it does.
Quick Recap
What to include in a three-location report
- Location names or clear descriptions, plus the rooms or areas sampled.
- Monitor models, sensor types, and the pollutants or conditions each device measured.
- Placement details, including height and proximity to windows, doors, vents, sources, or air cleaners.
- Start and end dates, sampling duration, data gaps, and relevant activities or events.
- Whether devices and procedures were consistent across locations and whether results are raw or adjusted.
- Known device limitations, age, and maintenance information, along with a careful distinction between measured trends and conclusions.
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