Indoor air quality monitoring and sensors field guide for HVAC

What IAQ monitoring is, and why the value is the response

Indoor air quality monitoring is the continuous, real-time measurement of the air people breathe inside a building. It is three things working together: the sensors that take the readings, the dashboard that shows them, and the alerts that tell someone when a number crosses a line. The hardware is the easy part. A box on the wall reading carbon dioxide, particulate, and humidity every minute is now cheap enough to put in every zone.

What that box is worth depends entirely on what happens after it reads high. Monitoring tells you when the air is bad so someone can do something about it: open the outdoor air dampers, step up the filtration, or send a technician to find the source. A sensor that reads 1,400 ppm of CO2 in a packed conference room at 2 p.m. and triggers no response is just a number on a screen. The value is the response, not the sensor.

So the real work is not picking a sensor. It is choosing what to measure for this building, placing and calibrating the sensors so the readings are true, building alerts that reach the person who can act, and wiring the data to a response, usually through the building automation system. Monitoring watches the air all the time. When it flags a problem it cannot explain, that is where a one-time IAQ investigation takes over, and the two jobs are covered in the companion guides cross-linked here.

Why a sensor nobody acts on is just a number

The most expensive mistake in IAQ monitoring is collecting data nobody uses. A building gets a wall of green-and-red tiles in the lobby, the dashboard looks impressive at the ribbon cutting, and six months later the CO2 in the east wing runs over 1,500 ppm every afternoon and nothing changes, because no alert reaches anyone with their hand on the dampers.

Tie every parameter you monitor to a specific action before you install the first sensor. High CO2 means add outdoor air. High particulate means step up filtration or recirculate against bad outdoor air. A combustion reading means investigate now. If you cannot name the response for a parameter, you do not need to monitor it yet, because the reading will not change anything.

This is the same lesson the fault detection and diagnostics world learned the hard way, and it is worth borrowing here. A building can be covered in sensors and still waste energy and run uncomfortable, because the data sits in a historian that nobody reads. IAQ monitoring fails the same way. The fix is to design the loop first, the reading to the alert to the responsible person to the action, and treat the sensor as the cheapest link in it.

What is the difference between IAQ monitoring and an IAQ investigation?

Monitoring is continuous and it watches the air all the time. An investigation is a one-time hunt that diagnoses a specific complaint. They are different jobs, and confusing them is a common and costly error.

Monitoring flags. It tells you that CO2 ran high in this zone every afternoon last week, or that particulate spiked the morning the wildfire smoke arrived. It is good at the question when, and decent at where, because it is always running and it remembers. What it cannot do is tell you why the air is bad in a way a sensor was never built to detect. A monitor reading normal does not clear a real complaint, and a monitor reading high does not name the source.

An investigation diagnoses. When someone reports headaches and the monitor shows nothing unusual, or shows a problem it cannot explain, that is when you start with the people, the complaint pattern, and the building, walk it with your eyes and nose, and bring the right instruments to confirm a hypothesis. The investigation finds the moisture behind the wall, the exhaust short-circuiting into the intake, the off-gassing material. See the indoor air quality investigation and testing guide for that work. Monitoring is the watchman. The investigation is the detective. You want both, and you call the detective when the watchman raises a flag the dashboard cannot resolve.

What should you monitor?

The parameters worth measuring depend on the building and the concern, but a short list covers most commercial spaces. Carbon dioxide is the single most useful reading because it stands in for ventilation. Particulate matter, PM2.5 and PM10, covers smoke, dust, and what gets through the filters. Total volatile organic compounds catches off-gassing and cleaning chemicals. Temperature and relative humidity cover comfort and the mold risk. Carbon monoxide is a separate, life-safety case. Radon is a long-term concern in some buildings and regions.

Match the set to the space. An open office lives and dies on CO2 and comfort. A building near a freeway or in wildfire country needs particulate. A space with new construction, fresh furniture, or heavy cleaning watches TVOC. A building with fuel-fired equipment, attached garages, or loading docks treats carbon monoxide as non-negotiable. Do not monitor a parameter because the sensor came with the package. Monitor it because you have a response for it.

The thresholds below are common reference points drawn from ASHRAE guidance and the monitoring programs that publish targets, not universal limits. The applicable ASHRAE standard, the project specification, and any certification program you are pursuing control the numbers that actually apply to your building.

What does a CO2 sensor actually tell you?

A CO2 sensor tells you whether the ventilation is keeping up with the people in the room. That is its real job, and it is the reason CO2 is the most useful single IAQ reading in an occupied building. People exhale carbon dioxide at a predictable rate. When fresh outdoor air is not coming in fast enough per person, exhaled CO2 builds up, and the concentration climbs. So CO2 is a cheap, direct proxy for the question that actually matters: is enough outdoor air reaching these people.

Outdoor air sits around 400 to 420 ppm. Indoors, a well-ventilated occupied space commonly holds somewhere near 800 to 1,000 ppm, and many programs and operators use a figure in that band as a working target. Be careful how you cite it. ASHRAE Standard 62.1 has not contained an indoor CO2 limit for decades, and no current ASHRAE standard sets one. ASHRAE guidance frames it as a difference, recommending the indoor level run no more than roughly 700 ppm above outdoor, and public-health guidance has pointed to readings under about 800 ppm as a sign of good ventilation. Treat any single number as a target you chose, not a code limit.

What a CO2 reading does not tell you is whether the air is otherwise clean. CO2 tracks bioeffluents and ventilation. It says nothing about particulate, VOCs, formaldehyde, radon, or combustion gases. A room can read a perfect 600 ppm and still be full of wildfire smoke or solvent vapor. CO2 is the ventilation gauge, not the air-quality verdict.

CO2 as the input to demand-controlled ventilation

The closed loop that makes CO2 monitoring pay is demand-controlled ventilation. Instead of running a fixed amount of outdoor air all day, the building automation system reads zone CO2 and modulates the outdoor air dampers to the actual occupancy: more fresh air when CO2 rises, less when the room empties out. That is monitoring tied directly to a response, which is exactly what the data is for.

The payoff is energy. Conditioning outdoor air costs money, and ventilating an empty conference room at full design rate all afternoon is waste. Tying outdoor air to measured CO2 commonly cuts ventilation energy by a meaningful margin while holding or improving air quality, because the air arrives when the people do. The balance to protect is the floor. Demand control should reduce excess ventilation, never starve a space below the minimum outdoor air the design and ASHRAE 62.1 call for.

When this loop misbehaves, it shows up as a controls and sequencing problem, not a sensor problem. A drifted CO2 sensor drives the dampers to the wrong position, a stuck damper ignores a correct reading, and a poorly tuned sequence hunts. That is fault detection territory. The building-automation fault detection and diagnostics guide cross-linked here covers reading the trends to catch a DCV loop that is fighting itself, and the same expected-versus-actual logic applies to outdoor air.

Particulate: PM2.5, PM10, and the filter

Particulate sensors measure the small stuff suspended in the air, reported as PM2.5 for fine particles and PM10 for the coarser fraction. PM2.5 is the health number people watch, because particles that small get deep into the lungs. Indoors, the particulate that matters comes from a few sources: wildfire smoke and traffic infiltrating from outdoors, dust kicked up by activity and construction, and whatever the filters fail to catch.

The response to a particulate reading is filtration and air handling. When indoor PM2.5 climbs because outdoor air is bad, the move is to recirculate more, tighten the building, and lean on better filters rather than pulling the smoke straight in through the economizer. When indoor particulate climbs while outdoor air is clean, the source is inside, and the monitor has done its job by pointing you at a filter that is loaded or bypassing, or an activity generating dust. Common targets for indoor PM2.5 run from roughly 9 to 35 ug/m3 depending on the program, with the lower end tracking EPA's annual PM2.5 standard that dropped to 9 ug/m3 in 2024 and the higher end matching 24-hour program limits like RESET, but the applicable standard and the project goal set the line.

Particulate is also where sensor grade bites hardest. Almost all affordable PM sensors are optical, and optical sensors estimate mass from how particles scatter light. They tend to read true for trend and direction but can over-report in absolute terms, especially in smoke, where the particle size and composition differ from what the factory calibration assumed. Use them to see the spike and drive the response. Lean on the manufacturer's correction and a reference instrument when you need a defensible absolute number.

TVOC: the chemical proxy

Total volatile organic compounds is a single index that lumps together the gaseous organic chemicals in the air: solvents, adhesives, cleaning products, fresh paint, new furniture and carpet off-gassing, printers, and the rest. A TVOC reading climbs when one or more of those is present in quantity. It is useful for catching a step change, the spike after the floors are stripped and waxed, the load that builds in a freshly renovated suite before it airs out.

Understand what TVOC is not. It is a proxy, not a measurement of any specific chemical. The sensor responds to a mixed bag of compounds with different sensitivities, so a high TVOC tells you something organic is off-gassing, not what it is or whether it is harmful at that level. Two readings of the same TVOC value can mean very different things. The number flags a condition. It does not diagnose a hazard.

So the response to high TVOC is to ventilate and, if it persists, investigate the source rather than to read a health risk off the index. If a TVOC pattern points at a real exposure concern, that is a question for an industrial hygienist with the instruments to speciate the compounds, which is investigation work, not monitoring.

Temperature and humidity: comfort and the mold line

Temperature and relative humidity are the comfort parameters, and they generate more complaints than any contaminant. They are worth monitoring because they are easy to read accurately, they drive the calls that reach the facility desk, and humidity carries a real risk beyond comfort.

Humidity is the one to watch. Keep indoor relative humidity in roughly the 30 to 60 percent band, and ideally nearer 30 to 50 percent. Too dry, below about 30 percent, and you get static, dry eyes, and irritated airways. Too wet is the bigger hazard. Sustained relative humidity above about 60 percent moves you into mold territory, because surfaces and materials hold enough moisture for growth, and condensation on cold surfaces makes it worse. ASHRAE comfort guidance commonly caps the upper bound near 60 percent, and the project conditions control the target you hold.

The value of humidity monitoring is catching the slow drift before it becomes a mold remediation. A space that creeps to 65 percent and sits there for weeks is growing a problem you will pay for later. The reading is the cheap early warning. The response is managing the moisture, dehumidification, fixing the load, or finding the intrusion, which on a persistent or hidden problem is again investigation work.

Carbon monoxide is life-safety, not comfort

Carbon monoxide sits apart from every other parameter in this guide, and it has to be treated that way. CO is a combustion product. It comes from anything burning fuel: furnaces and boilers with a cracked heat exchanger or a blocked flue, vehicles in attached garages and at loading docks, generators, and unvented appliances. It is colorless and odorless, and it kills before anyone complains, which is exactly why it cannot be lumped in with comfort monitoring.

Do not confuse an IAQ carbon-monoxide reading with a CO alarm. A life-safety CO alarm is a listed device built and certified to sound at concentrations and exposure times set to protect occupants, and where one is required, the listed alarm governs. An IAQ monitor that happens to read CO is useful for seeing low-level trends and catching a problem early, but it is not a substitute for the listed alarm or for the requirements that apply to it. Confirm what the applicable codes and the AHJ require, and provide that, separately from any comfort dashboard.

If a CO reading is real and high, the response is not a work order in a queue. It is people out, ventilation up, and the combustion source found and shut down. This is the one parameter where you act first and analyze second.

What makes a good IAQ sensor?

A good IAQ sensor reads true, in the right place, and keeps reading true over time. Two things decide whether the data is worth anything: the grade of the sensor and where it sits. A cheap, uncalibrated sensor in a bad spot produces confident numbers that are wrong, and wrong numbers are worse than no numbers, because people act on them.

Grade is about how the sensor measures. The good CO2 sensors use NDIR, nondispersive infrared, which measures carbon dioxide directly by how much infrared light the gas absorbs. The cheap ones often report an estimated CO2 derived from a VOC sensor, which is not a real CO2 measurement and drifts with anything else in the air. For CO2, NDIR is the line between a useful reading and a guess. Particulate sensors are optical and vary widely in how well they hold calibration. Match the grade to the use: a reference-grade instrument for compliance and commissioning, a solid building-grade monitor for operations, and consumer devices for awareness only.

Placement is the other half, and it is covered in its own section below. The short version: the sensor has to sample the air people actually breathe, not the air by a door or a diffuser. The best sensor in the wrong spot lies as confidently as the worst one.

Sensor grade: consumer, building, and reference

Sensors span a wide range of accuracy and price, and the trade is real. At the bottom are consumer devices fine for personal awareness and trend. In the middle are building-grade monitors meant for permanent installation and operations, accurate enough to drive a response and to satisfy many monitoring programs. At the top are reference-grade and regulatory instruments used for compliance and for calibrating everything else.

Garbage sensor in, garbage data out. A low-cost CO2 sensor can read tens or hundreds of ppm off out of the box, and the estimated-CO2 type drifts further. Low-cost optical PM sensors correlate well with the trend but commonly over-report mass, so the spike is real even when the absolute number is not. Knowing the failure mode of the grade you bought is part of reading it honestly.

Match the grade to the decision the data drives. If a reading sets a damper, satisfies a certification, or supports a complaint response, it needs a grade and a calibration record to stand behind it. If it only colors a tile in a lobby display, a cheaper device is fine, as long as nobody mistakes it for a defensible measurement. The manufacturer's published accuracy spec, and its stated calibration interval, are the numbers to design around, not the marketing.

Calibration and drift: why an uncalibrated sensor lies

Every IAQ sensor drifts, and a drifted sensor reads wrong with full confidence. On NDIR CO2 sensors the infrared source slowly ages and loses intensity, which the sensor reads as rising CO2, so an old uncalibrated unit creeps high over months and years. Optical PM sensors foul and age. An uncalibrated sensor is worse than no sensor, because it drives wrong responses and erodes trust in the whole system.

Many CO2 sensors handle drift with automatic baseline correction, ABC, an algorithm that assumes the lowest reading over a rolling window, often the lowest in each 24-hour period, equals fresh outdoor air near 400 to 420 ppm, and recalibrates to that floor. ABC works well in spaces that empty out regularly, because the room actually reaches outdoor levels overnight. It fails quietly in spaces that are occupied around the clock and never drop to baseline, where it can drag readings erroneously low. Know whether ABC is on and whether your space ever hits the floor it assumes.

Plan calibration as maintenance, not a one-time setup. Follow the manufacturer's recommended interval and method, verify against a reference or a known gas where accuracy matters, and keep the record. The hard rule: if you cannot say when a sensor was last calibrated and whether it has held, you cannot defend the number it is showing.

Where should IAQ sensors be placed?

Place IAQ sensors in the breathing zone of a representative spot in the occupied space, away from anything that gives a false reading. Roughly 3 to 6 ft off the floor puts the sensor where people actually breathe rather than at the ceiling or the floor, where conditions differ. The goal is air that represents what occupants experience, not a convenient mounting location.

Avoid the spots that lie. A sensor next to a supply diffuser reads the conditioned supply air, not the room, so CO2 looks artificially low and the dampers get the wrong signal. A sensor by a door, an operable window, or a high-traffic corridor reads the churn, not the zone. A sensor in a dead corner with no air movement lags and misses the room. A CO2 sensor mounted where someone's breath hits it directly, like right beside a workstation or a podium, reads that one person, not the space.

Think per-zone, not per-building. Different zones ventilate and load differently, so one sensor in the lobby tells you almost nothing about the packed third-floor conference room. Put sensors where the people and the risk are, in numbers that match how the building actually breaks into zones, and put the CO2 sensors specifically where you would set the DCV control point. A sensor in the wrong place produces honest-looking data about the wrong air.

Dashboards and alerts that get acted on

The dashboard exists to drive action, not to look impressive. Real-time tiles, trend charts, and history all have a place, but the part that earns its keep is the alert: a threshold alarm that fires when a parameter crosses the line you set and reaches the person who can respond. A hundred charts nobody opens is worse than three thresholds wired to a phone.

Build alerts that someone will actually answer. Set thresholds that mean something for this building, route them to the facility manager or the controls team on a channel they watch, and tune them so they fire on a real problem rather than every minor wiggle. An alert system that cries wolf gets muted, and a muted alert is a sensor nobody acts on. Trends matter too, because the slow creep, humidity climbing week over week or CO2 baselines rising as occupancy grows, is the problem you want to catch before it becomes a complaint.

Tenant-facing displays are a different audience with a different job. A lobby or floor display showing CO2, particulate, and humidity builds confidence and transparency, and post-2020 that has real value to occupants. Keep the public view simple and the operations view actionable. They are not the same screen.

Closing the loop: tie the data to a response

This is the part that decides whether the whole system was worth installing. Every parameter you monitor needs a defined response, and the strongest responses are automated through the building automation system so they happen without waiting on a human. High CO2 drives more outdoor air through demand-controlled ventilation. High particulate steps up filtration or shifts the building toward recirculation against bad outdoor air. A combustion or life-safety reading triggers an alarm and an investigation. Anything the BAS cannot resolve becomes a work order with a name on it.

The loop is detect, respond, and verify in the data that the response worked. A reading goes high, the system or a person acts, and the trend confirms the air came back. If it does not, the response failed or the diagnosis was wrong, and that is a signal in itself. This is the same closed loop the fault detection and diagnostics guide describes for energy and equipment faults, applied to the air, and the BAS is where the two meet.

Where monitoring runs out of road, hand off cleanly. A persistent reading the dashboard cannot explain, a complaint the sensors do not reflect, or a health concern is the point where you stop adjusting dampers and call an investigation. Monitoring closes the loops it can and flags the ones it cannot. Knowing which is which is the skill.

The standards and what is driving the demand

Continuous IAQ monitoring went from nice-to-have to expected after 2020, and a handful of standards and programs now frame it. ASHRAE Standard 62.1 is the ventilation baseline, defining how much outdoor air a space needs, which is the rate CO2 monitoring effectively checks against. It sets ventilation requirements, not a CO2 limit, a distinction worth getting right when you cite it.

ASHRAE Standard 241, Control of Infectious Aerosols, published in 2023, is the newer driver. It sets requirements for equivalent clean airflow, the combined effect of ventilation, filtration, and air cleaning, to reduce disease transmission risk, and it includes an infection risk management mode for when added protection is needed. Monitoring is how a building shows it is delivering that air. On the certification side, the WELL Building Standard and RESET Air both require continuous monitoring of a core parameter set. RESET Air calls for constant measurement of PM2.5, TVOC, CO2, temperature, and humidity, with published targets such as PM2.5 at or below 35 ug/m3 and CO2 below 1,000 ppm, and tighter high-performance levels. WELL's air-monitoring feature asks for building-grade monitors reading CO2, PM2.5, and TVOC at intervals no longer than an hour.

Cite these by what they actually require and verify the current edition. The numbers in certification programs are theirs and change between versions, the ASHRAE standards are adopted and amended by jurisdiction, and the project specification controls which apply to your building.

Why owners are buying it: tenant demand and transparency

The market reason monitoring sells is the tenant. Since 2020, occupants expect to know the air they are breathing is being watched, and tenants signing commercial leases increasingly ask about it. A lobby display showing live air quality, and a building that can produce a clean record of its CO2, particulate, and humidity, is a leasing and retention argument.

Transparency is the product as much as the air itself. Showing the data publicly signals that the building takes it seriously, which feeds recruiting and wellness narratives that matter to the employers who sign the leases. A WELL or RESET certification is partly a marketing asset for exactly this reason, and the continuous monitoring is what backs the claim.

For the owner, that turns an operating cost into a differentiator, which is why the budget shows up. The risk is buying the display and skipping the response. A lobby screen with no loop behind it is theater, and a sophisticated tenant or a certification auditor sees through it fast.

What can't IAQ monitoring tell you?

Monitoring measures what its sensors measure, and nothing else. A typical monitor covers CO2, particulate, TVOC, temperature, and humidity, and maybe CO. It does not see formaldehyde, radon, mold spores, specific solvents, combustion gases it lacks a sensor for, or any contaminant nobody installed a sensor to catch. There is no single box that watches every contaminant, so a normal dashboard never means the air is clean of everything. It means the parameters you chose to watch are within range.

Monitoring flags, it does not diagnose. A high reading tells you a condition exists, not its source or its health significance. A TVOC spike does not name the chemical. A particulate spike does not tell you whether it came from outdoor smoke or an indoor source. The diagnosis is a separate job, the IAQ investigation cross-linked here, run by someone who starts with the people and the building and brings the right instruments.

The quiet danger is the false sense of safety. A wall of green tiles reassures everyone while an unmonitored contaminant goes undetected, or while an uncalibrated sensor reads green because it drifted, not because the air is good. Calibration, honest scope, and knowing where monitoring ends and investigation begins are what keep the dashboard truthful. Treat it as an always-on smoke detector for a handful of parameters, not a verdict on the air.

What to document

The data is only as good as the record behind it, and the record is what lets you defend a reading and prove a building delivered clean air. Capture the parameters you monitor and why, the grade and model of each sensor, its location, and its calibration history, the thresholds and the response tied to each, and the trends over time. That history is the compliance record for a certification, the evidence in a complaint, and the input that ties the system back to the BAS and to a field tool like FieldOS for tracking responses and calibration tasks.

Keep the calibration log especially tight, because it is the first thing questioned when a number is challenged. A reading without a known calibration date is a number you cannot stand behind. The table below is the minimum a building should be able to produce on request.

Common mistakes

• Collecting IAQ data nobody acts on, so a high reading changes nothing.
• Buying cheap, uncalibrated sensors that read confidently wrong, including estimated-CO2 devices sold as CO2 sensors.
• Placing a sensor by a diffuser, a door, or in a dead corner, so it reads the wrong air.
• Monitoring with no response tied to the ventilation or filtration, so the dashboard is theater.
• Mistaking monitoring for diagnosis and expecting a sensor to name a source it was never built to detect.
• Ignoring CO2 as the ventilation proxy, or treating a single CO2 number as an enforceable code limit.
• Leaving automatic baseline correction on in a space occupied around the clock that never reaches outdoor baseline.
• Treating an IAQ CO reading as a substitute for a listed life-safety CO alarm.

Field checklist

Standards and references

ASHRAE is the body to cite for the air itself. Standard 62.1 sets the ventilation rates that CO2 monitoring effectively checks, and it does not set an indoor CO2 limit, so frame CO2 targets as design choices, not code. Standard 241, Control of Infectious Aerosols, published in 2023, sets equivalent clean airflow requirements combining ventilation, filtration, and air cleaning, and an infection risk management mode. Both are adopted and amended by jurisdiction, so confirm the edition that applies before citing a number on a submittal.

For the sensors, the manufacturer is the authority. Accuracy specifications, the measurement method, the recommended calibration interval and procedure, and the behavior of automatic baseline correction come from the maker's documentation, and they govern what a given device can and cannot defend. Do not assume a published accuracy without checking the model.

The certification programs publish their own monitoring requirements and targets. The WELL Building Standard and RESET Air both require continuous monitoring of a core parameter set with specified intervals and thresholds, and those numbers are theirs and change between versions. For diagnosis, the authority is a qualified IAQ professional or industrial hygienist, because monitoring flags a condition and the investigation names the cause. The three points to hold onto: the value is the response, not the sensor; use calibrated, grade-appropriate sensors placed in the breathing zone; and use CO2 as the ventilation proxy and tie it to the BAS.

Units and terms

IAQ monitoring borrows units and terms from several places, and the same concept can read differently across a sensor sheet, a certification program, and an HVAC spec.

Carbon dioxide and carbon monoxide are reported in parts per million, ppm. Particulate is reported as a mass concentration in micrograms per cubic meter, ug/m3, split into PM2.5 and PM10 by particle size. TVOC is an index, often in ppb or ug/m3, that represents a mixed group of compounds rather than one chemical. Relative humidity is a percentage. The terms below are the ones that cause the most confusion on the job.

IAQ monitoring vs investigation

Monitoring is continuous, real-time measurement that flags when air is bad; an investigation is a one-time hunt that diagnoses the cause of a complaint

CO2 as ventilation proxy

Carbon dioxide stands in for outdoor air per person, because exhaled CO2 builds up when ventilation cannot keep pace with occupancy

PM2.5 / PM10

Particulate matter 2.5 and 10 microns and smaller, the fine and coarse fractions, reported in ug/m3

TVOC

Total volatile organic compounds, a single proxy index for a mix of gaseous organics, not a measurement of any one chemical

Demand-controlled ventilation (DCV)

Controlling outdoor air to measured CO2 or occupancy, adding fresh air when CO2 rises and cutting it when a space empties

NDIR

Nondispersive infrared, the method good CO2 sensors use to measure carbon dioxide directly by infrared absorption

Sensor grade / calibration / drift

Grade is the accuracy class from consumer to reference; calibration corrects the reading to a known value; drift is the gradual error a sensor develops as it ages

ABC

Automatic baseline correction, an algorithm that recalibrates a CO2 sensor by assuming its periodic low reading equals fresh outdoor air

ASHRAE 241

ASHRAE Standard 241-2023, Control of Infectious Aerosols, which sets equivalent clean airflow requirements to reduce disease transmission risk

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