WELL Building Standard and air quality: what monitoring can show

In the WELL Building Standard (WELL v2, standard text dated Q2 2025, published by the International WELL Building Institute, IWBI), air quality monitoring appears in five Air features, and only two of them are preconditions. A01 Air quality and A03 Ventilation design are preconditions. A05 Enhanced air quality, A06 Enhanced ventilation design and A08 Air quality monitoring and awareness are optimizations. A CO2 threshold appears in A03 and A06, and none in A01. The version matters: this article uses the Q2 2025 text, and the Performance Verification Guidebook is a separate document.

What the WELL Building Standard text requires

• A01 Air quality (precondition). Part 1 sets particulate matter at PM2.5 of 15 µg/m³ or lower and PM10 of 50 µg/m³ or lower under its first option. Part 2 offers laboratory-tested organic gases or total VOC of 500 µg/m³ or lower. Part 3 sets carbon monoxide at 10 mg/m³ (9 ppm) and ozone at 100 µg/m³ (51 ppb). CO2 is not part of A01.
• A03 Ventilation design (precondition). Option 4, ventilation monitoring, asks for CO2 of 900 ppm or less, or not more than 500 ppm above outdoor levels, in occupiable spaces. It is one route among several, alongside mechanical design to ASHRAE 62.1-2010 or a more recent version (or another listed guideline such as EN 16798-1) and natural ventilation options.
• A05 Enhanced air quality (optimization, up to 4 points). PM2.5 of 12 µg/m³ or lower with PM10 of 30 µg/m³ or lower earns one point under particulate matter, and 10 and 20 µg/m³ earn two. Inorganic gases add a point at carbon monoxide of 7 mg/m³ (6 ppm) and nitrogen dioxide of 40 µg/m³ (21 ppb).
• A06 Enhanced ventilation design (optimization, up to 3 points). The monitoring option asks for CO2 of 750 ppm or less, or not more than 350 ppm above outdoor levels, and is worth 2 points. The demand control ventilation option uses 900 ppm (500 above outdoor) for one point and 750 ppm (350 above outdoor) for two.
• A08 Air quality monitoring and awareness (optimization, up to 2 points). Part 1 asks for monitors whose sensors measure at least three of seven parameters in occupiable spaces: PM2.5 or PM10, CO2, carbon monoxide, ozone, nitrogen dioxide, total VOC and formaldehyde. Part 2 adds a point only if Part 1 is met: display screens, or a website or app with signs, at least one display or sign per 500 m² of regularly occupied space, updated at least once every 15 minutes.

Which features apply to a given project depends on its type (owner-occupied or WELL Core) and parameters in the WELL documents, and WELL Core uses different point values than owner-occupied projects. The labels and points above are the ones in the standard text.

How the data is checked

Verification rules for the WELL Building Standard sit in the WELL Performance Verification Guidebook (version 2025-03, published 13 August 2025), not in the standard text. For continuous monitoring it sets:

• CO2 sensors: non-dispersive infrared (NDIR), range 400 to 5,000 ppm, accuracy of ±50 ppm plus 5% between 400 and 2,000 ppm
• PM2.5 sensors: optical particle counters, accuracy of ±5 µg/m³ plus 20% between 1 and 100 µg/m³
• readings at intervals of 15 minutes or less, with wall-mounted monitors 1.1 to 1.7 m above the floor and at least 1 m from interior doors and windows
• one monitor per 325 m² for projects under 3,250 m² (minimum two), one per 500 m² from 3,250 to 25,000 m² (minimum 10) and one per 1,000 m² above that (minimum 50)
• at least 90% of each sensor's data from regularly occupied hours within the threshold, with missing data counting against, and at least one calendar month of data for an initial award

On calibration, the A08 text says proof of calibration or replacement is submitted annually, and an IWBI support article gives "annually for air quality parameters" as the example frequency for monitor-deployment maintenance reports. The Guidebook's hardware section says air quality sensors are recalibrated or replaced every three years, with documentation attesting to it every three years. The documents do not say how these fit together, so check the current versions of all three.

What continuous monitoring data can show

Logged CO2 can show how well a room's ventilation kept up with the people in it, hour by hour. REHVA's COVID-19 guidance (version 4.1) states that CO2 readings describe the outdoor ventilation rate adequately under normal occupant density, and that high CO2 indicates poor ventilation without question. ASHRAE says readings far above expected ranges may indicate the ventilation system is not functioning properly. A month of 15-minute data can show which rooms run high, at what times of day, and whether a change to the system moved the numbers. A threshold like the one in A03 can be checked against that record; the Guidebook tests the share of a sensor's readings that stay within the limit.

For what CO2 numbers mean in an office, see What is a good CO2 level in an office?. Where the sensor sits changes what it reads, which Where to place CO2 sensors for ventilation control covers.

What it cannot show

ASHRAE says indoor CO2 concentrations do not provide an overall indication of indoor air quality, though they can be a useful tool. Readings below a threshold do not assure acceptable air quality, because temperature, humidity, particulate matter and gas pollutants also matter. ASHRAE also notes that CO2 concentration is unaffected by filtration. A CO2 monitor says nothing about PM2.5, formaldehyde, ozone or radon, which A01 and A05 cover. For the wider picture, see Indoor air quality in offices, how to measure it.

Sensor accuracy, location and calibration are, in ASHRAE's words, "all critical for drawing meaningful inferences." A monitor on a wall at 1.1 to 1.7 m reports the air at that spot. ASHRAE also notes that the evidence on CO2 and health, learning and work performance is inconsistent, and that its ventilation standards 62.1 and 62.2 contain no CO2 limits. The CO2 thresholds above come from the WELL Building Standard text, not from ASHRAE 62.1.

Whether a given data set earns a point, meets a precondition or supports a certification level is decided by IWBI and its verification process.