Indoor air quality investigation and testing field guide for HVAC

What an IAQ investigation is, and why you start with the building

An indoor air quality investigation is a structured search for what is making the air in a building uncomfortable, smelly, or making people feel unwell, run by working backward from the complaint to a cause. The instinct when someone reports headaches or a stuffy room is to grab an air meter and start taking numbers. That is the wrong first move. A meter reading means nothing until you know what you are chasing, and a building full of normal-looking numbers does not clear a real complaint.

The work starts with three things: the people, the complaint pattern, and the building. Who feels it, what they feel, where in the building, and when. Then you walk the building and use your eyes, your nose, and the HVAC drawings before you unpack a single instrument. Most air complaints trace back to a short list of causes, and inadequate ventilation sits at the top of it, with moisture close behind. The meter is there to confirm a hypothesis you have already formed, not to go fishing.

The one exception to going slow is carbon monoxide. CO is the contaminant that kills before it complains, so it gets checked first, on every job, before the methodical part begins. Everything else in this guide is the patient hunt. CO is the fast safety check that comes ahead of it.

Start with the people and the building, not the meter

The order of operations is the whole skill. Start by collecting the complaint: who is affected, what symptoms or nuisances they report, which rooms or floors, and the timing. Timing is the strongest clue most people walk past. A complaint that peaks Monday morning points at something that built up over a closed weekend. One that tracks the afternoon sun points at a solar and ventilation load. One that started the week the new carpet went in points at off-gassing. The pattern often names the cause before any instrument comes out.

Pull the building history alongside it. Recent renovation, a roof leak, a new tenant, a changed cleaning product, an HVAC repair, a parking change near the air intake. The EPA's Building Air Quality guidance frames the investigation this way for a reason: an interview and a records review are faster and cheaper than a testing program, and they tell you where to point the testing if you end up needing it.

Resist the pressure to just test the air. Broad air sampling with no hypothesis produces a stack of numbers nobody can interpret and a bill nobody can justify. A general, systematic walkthrough beats a shotgun sampling plan almost every time. Test to confirm a specific suspicion, not to look busy.

Carbon monoxide gets checked first, every time

Before the methodical part starts, check for carbon monoxide. CO is colorless, odorless, and the one IAQ contaminant that can put people in the ground while you are still interviewing them about a stuffy room. If there is any combustion in or near the building, a furnace, a boiler, a water heater, a gas range, an attached garage, a generator, or a delivery dock, CO is on the table and it goes to the front of the line.

Carry a CO meter and read the occupied space first, then near each combustion appliance and its venting. OSHA's permissible exposure limit is 50 ppm as an 8-hour average, and effects begin well below the level a basic residential alarm will sound on. A reading that climbs near a furnace closet, a return pulling air from a garage, or spillage at a draft hood is a stop-work finding, not a data point for the report.

If you find CO, the investigation pauses. You ventilate, you find and shut down the source, and you get people out if the level warrants it. The slow, systematic hunt is for everything else. CO is the one that does not wait, and treating it like just another reading on the list is how an investigator gets someone hurt. Confirm action levels against OSHA and the appliance manufacturer, and when combustion is involved, a combustion-analysis check belongs in the scope.

Why indoor air quality is worth investigating

People spend most of their lives indoors, so the air inside a building does more to their comfort and health than the outdoor air ever will. Poor IAQ shows up as the symptoms a building manager hears about constantly: headaches, tiredness, stuffy or dry sensations, irritated eyes and throats, and a general sense that a room is hard to work in. Those complaints are real even when no single measurement is alarming.

There is a performance cost behind the comfort cost. Research links better ventilation and lower indoor pollutant levels to fewer symptoms, less absence, and better cognitive performance, which is why employers and owners care beyond simple courtesy. A floor full of people who cannot concentrate after lunch is a productivity problem with an air cause.

And there is liability. An ignored complaint that turns into a documented health pattern, or a mold problem that festers behind a wall, becomes an expensive problem with a paper trail. The investigation exists to find the cause early, fix it, and leave a record that shows the building was managed responsibly. None of that requires diagnosing anyone. It requires finding and correcting the building condition.

Inadequate ventilation is the most common cause

More IAQ complaints trace back to inadequate ventilation than to anything else. The EPA groups the major causes into inadequate ventilation, indoor chemical sources, outdoor chemical sources, and biological contaminants, and not enough outdoor air is the one that turns up most often. When a building does not bring in enough fresh outdoor air, everything people and the building emit, carbon dioxide, body odor, off-gassing from finishes, accumulates, and the space goes stale.

The history is worth knowing. Older ventilation practice supplied roughly 15 cfm of outdoor air per person. After the 1970s energy crunch, that got cut toward 5 cfm to save conditioning energy, and a wave of stuffy, complaint-ridden buildings followed. Ventilation rates came back up, but the failure mode never went away: a building starved of outdoor air to save energy is the classic stale-building setup.

The fix for this cause is air, not a gadget. More outdoor air, or outdoor air that is actually being delivered when it was supposed to be, clears most stuffiness complaints. How much air a space needs, how it is calculated under ASHRAE Standard 62.1, and how to verify it is delivered is the Anvilfield ventilation rate and outdoor air guide. For the investigation, the lesson is to suspect ventilation first and confirm whether the design rate is reaching the space.

The building walkthrough

The walkthrough is where an experienced investigator earns the fee, because most causes are visible, smellable, or audible to someone who knows what to look for. Walk the complaint area first, then the spaces around it, then the mechanical rooms and the path the air takes to get there. You are building a picture of the sources, the HVAC, and where moisture and odors are coming from.

Use your senses deliberately. Look for water stains, condensation, standing water in drain pans, visible mold, dust on diffusers, and blocked returns. Smell for the musty note that means moisture and microbial growth, the solvent note that means off-gassing or a cleaning product, the sewer-gas note that means a dry trap. Feel for airflow at the registers, because a register with no air moving is a ventilation problem you can find with your hand.

Look at the HVAC the way an inspector does. Where is the outdoor air intake, and what is near it, a loading dock, a trash area, a cooling tower, a parking garage exhaust. Are the filters loaded or bypassed. Is the outdoor air damper actually open. Note what occupants have done to cope, the propped doors, the desk fans, the taped-over diffusers, because those are a map of where the system is failing the people in the room.

What does high CO2 in a building mean?

High carbon dioxide indoors almost always means too little outdoor air for the number of people in the space, not that CO2 itself is poisoning anyone. At the levels seen in normal occupied buildings, CO2 is not a toxin. It is a marker. People exhale it steadily, so the indoor concentration above the outdoor level tracks how much outdoor air is being supplied per person. That makes it the single most useful ventilation-adequacy reading an investigator carries.

The number most people work to is roughly 1,000 to 1,100 ppm indoors, often stated as about 700 ppm above a typical outdoor level near 400 ppm. The basis is comfort, not toxicity: around 1,000 ppm a noticeable fraction of people start finding the air stale from human bioeffluents. Read a high CO2 as the ventilation not keeping up with the people, which is exactly the stuffy-room complaint, and confirm the setpoint and basis against the adopted ASHRAE standard, since the framing of indoor CO2 has been refined.

What high CO2 is not is a license to blame CO2 for headaches. It points at the ventilation rate, and that is what you go check. Low CO2 with complaints still present steers you away from a people-and-ventilation cause and toward a source or moisture problem instead.

Moisture is the root of mold and biological problems

Mold is a moisture problem wearing a biology costume. Mold spores are everywhere already, indoors and out, and they grow into a problem only where they find water and a surface to live on. Find the water and you have found the cause. Chase the mold without finding the moisture source and it comes back, because you treated the symptom and left the cause running.

On the walkthrough, hunt for the water: roof and window leaks, plumbing leaks, condensation on cold surfaces and ductwork, a cooling coil drain pan that is not draining, humidifiers, and chronically high indoor humidity. A musty smell with no visible growth usually means moisture and microbial growth hidden in a wall cavity, above a ceiling, or inside an air handler. The nose finds it before the eye does.

The fix is to stop the water and dry the materials, then remove what is contaminated. The EPA's guidance is clear that the moisture has to be corrected or the remediation fails. Identifying a specific mold species rarely changes the response, which is why authorities generally advise against routine air sampling for mold and toward finding and fixing the water. When the question becomes health exposure or a contested claim, that is industrial hygienist and laboratory territory, not a place to guess.

VOCs and off-gassing

Volatile organic compounds are gases that off-gas from materials and products: fresh paint, new carpet and adhesives, furniture, particleboard, solvents, cleaning supplies, printers, and stored chemicals. They are the cause behind the classic complaint that started right after a renovation or a furniture delivery and faded over weeks as the new materials cured. The solvent or chemical smell on the walkthrough is the tell.

Field instruments screen for total VOCs, reported as TVOC, which is the lumped sum of whatever the sensor responds to rather than a measurement of any one compound. TVOC is useful for finding a source and comparing one room to another, a high reading near the new casework and a low reading across the floor points you at the casework, but it is not a health number. There is no single agreed health-based TVOC limit, and a specific compound of concern, formaldehyde for example, has to be measured on its own.

The fix is source control and ventilation, in that order. Remove or seal the source where you can, and air the space out, with extra outdoor air during and after a renovation while materials cure. A particle filter does nothing for a gas, so a higher MERV is not the answer to an odor or off-gassing complaint. Where odor is the issue, the gas-phase filtration question is covered in the Anvilfield air filtration and MERV guide.

Combustion products and carbon monoxide

Combustion is the deadly corner of IAQ, and it is its own category because the failure can be fatal rather than merely uncomfortable. Any fuel-burning appliance, a furnace, boiler, water heater, gas range, space heater, or a vehicle or generator nearby, produces carbon monoxide and other combustion products, and a healthy appliance sends them up the flue. The danger is when they come back into the space.

Backdrafting and spillage are the mechanisms to understand. When a building runs negative, often because exhaust fans or a big kitchen hood pull more air out than makeup air replaces, the path of least resistance for makeup air can be down a flue, dragging combustion products back into the building. A blocked or disconnected vent, a cracked heat exchanger, or an appliance run in a too-tight space does the same. This is where the ventilation balance and combustion safety overlap, and it is why a negative building is more than a comfort problem.

Check it with a CO meter and, where combustion is suspected, a combustion analyzer at the appliance. A working CO alarm is the last line of defense, not the investigation, since alarms are set to protect against acute poisoning rather than to flag the low chronic exposure that still makes people sick. Confirm appliance venting, building pressure, and CO action levels against OSHA, the manufacturer, and the applicable fuel-gas code, and treat any spillage as a stop-work safety finding.

Particulates and PM2.5

Particulate matter is the solid and liquid material suspended in the air, sorted by size because size drives both where it comes from and how deep it goes into the lungs. PM10 is the coarse fraction up to 10 microns, dust, pollen, and lint. PM2.5 is the fine fraction up to 2.5 microns, combustion particles, smoke, and the soot that rides in with traffic and wildfire air. The fine fraction is the one health agencies track most closely because it penetrates deepest.

Indoors, particulate comes from outdoor air pulled in through the intake, from indoor sources like cooking, printers, and foot-traffic dust, and from a filter that is loaded, bypassed, or simply too low a MERV to catch the fine stuff. A complaint of visible dust settling on surfaces, or allergy-type symptoms, points at the particle side rather than the gas or ventilation side.

A handheld particle counter or an optical PM monitor gives a fast read and lets you compare indoors to outdoors and room to room, which is usually more informative than the absolute number off a low-cost sensor. If indoor PM tracks outdoor PM, the intake and the filtration are the lever. If indoor PM spikes on its own, you have an indoor source to find. Filter selection and the MERV-versus-static tradeoff is the Anvilfield air filtration and MERV guide.

Humidity, high and low

Relative humidity is a comfort and a contaminant problem at both ends, which is why it earns its own check. Too high, generally sustained above about 60 percent, and you are feeding mold, dust mites, and a clammy discomfort, plus condensation on cold surfaces that starts the moisture cycle. Too low, down in the teens and twenties in a dry winter, and you get dry eyes, irritated airways, static, and complaints that read like bad air but are really just dryness.

A commonly cited comfort and health range is roughly 30 to 60 percent, with many investigators aiming for 40 to 50 percent where the climate and the equipment allow it. Treat that as a target band, not a hard line, and confirm comfort conditions against ASHRAE Standard 55, which governs the temperature and humidity comfort zone. Temperature rides along with it, since a complaint of stuffiness is often as much a temperature and air-motion problem as a fresh-air problem.

Carry a thermo-hygrometer and read RH and temperature in the complaint area, not just at the thermostat. High RH points you back at moisture sources, drainage, and dehumidification. Low RH in winter points at over-ventilation or a lack of humidification. The reading sorts a vague the-air-feels-bad complaint into something you can act on.

The measurements and what each one tells you

The instrument kit for a first-pass IAQ investigation is small, and each tool answers a specific question. The mistake is carrying numbers around with no question attached. Read each parameter to test a hypothesis the walkthrough already raised.

Carbon dioxide tests whether ventilation is keeping up with the people. Carbon monoxide tests for the combustion safety hazard and gets read first. Temperature and relative humidity test comfort and the moisture setup. A particle monitor tests the dust and fine-particle side and is most useful as an indoor-to-outdoor comparison. A TVOC screen tests for off-gassing and chemical sources and points at a location rather than naming a compound. That short list resolves most complaints, and it tells you whether you need to escalate to targeted lab sampling.

The discipline is to interpret each reading against what it actually indicates and against the outdoor baseline, not against a single magic threshold. CO2 over the comfort guideline means ventilation. CO over an action level means stop. A TVOC spike in one room means a source there. Humidity out of band means moisture or dryness. None of these is a diagnosis of a person's health. They are the building's gauges, and they tell you which system to go fix.

The limits of cheap sensors

Low-cost IAQ sensors have gotten good enough to be useful and not good enough to be trusted blindly, and knowing the difference keeps you from chasing a number that is not real. Consumer and even prosumer TVOC and PM sensors are screening tools. They drift, they respond to interfering gases, and many report a relative index rather than a calibrated concentration. A TVOC display jumping when someone uses hand sanitizer nearby is the sensor working as designed, not a contamination event.

Particle sensors in the cheap tier are optical estimators that infer mass from a light-scatter count, and they can read high or low depending on particle type and humidity. CO2 is the better-behaved measurement in the affordable range, but even there a drifting or poorly mounted sensor misleads, which is why ventilation controls specify accuracy and recalibration. The honest use of any of these is comparison and trend: indoors versus outdoors, room versus room, before versus after, not a single absolute number read off the screen and dropped into a report as fact.

When a number has to stand up, for a health concern, a contested claim, or a specific contaminant, you move from screening to reference-grade instruments or laboratory analysis, run by someone qualified. Do not over-read a consumer sensor. It is a flashlight for finding the source, not a measurement that settles an exposure question.

Investigating the HVAC system

The HVAC system is where the most common IAQ causes live, so once the walkthrough points at ventilation or distribution, the system gets a real look. Three things matter most: whether outdoor air is actually being brought in, whether the filters are doing their job, and whether the air is reaching the people who are complaining.

Start at the outdoor air. Find the intake, confirm the outdoor air damper is open to at least its minimum position, and check that the design ventilation rate under ASHRAE 62.1 is actually being delivered, not just assumed. A unit balanced for supply air with nobody verifying the outdoor fraction is the single most common way a correctly designed building still breathes badly. The rate procedure and how to verify delivered outdoor air is the Anvilfield ventilation rate and outdoor air guide.

Then check the rest of the path. Filters loaded, bypassed, or the wrong MERV. An economizer stuck at minimum or hunting. Controls that command outdoor air to zero on a schedule nobody remembers setting. Diffusers and returns blocked or short-circuiting so the air never reaches the breathing zone. The HVAC investigation is mostly confirming that the system is doing what its design said it would, because the usual finding is that it is not.

The closed outdoor-air damper

If there is a single most common HVAC fault behind a stale building, it is an outdoor air damper that is shut or barely open. The damper saves energy when it closes, because outdoor air is a heating and cooling load, so there is constant pressure, deliberate and accidental, to keep it shut. A stuck linkage, a failed actuator, a minimum position set to zero, a control sequence that closes it overnight and never reopens, or a well-meaning operator who closed it to cut a bill, any of these starves the building while the fans run and everything looks normal.

The failure is quiet because the system still moves plenty of air. It is just the same recirculated air going around, carrying the building's own CO2 and odors with it, diluting nothing. High CO2 with strong airflow at the registers is the classic signature: lots of air, no fresh air.

Check the damper physically and check it under control. Confirm it opens to its minimum position when the unit calls for occupied ventilation, and measure the delivered outdoor air rather than trusting the commanded position. A damper that reads open at the controller and is mechanically stuck shut is a fault you only catch by looking at the blades and measuring the air.

Filters: loaded, bypassed, or wrong

Filters cause IAQ trouble three ways, and all three are common. A loaded filter that nobody changed on time adds static, drops airflow, and can blow accumulated dust through if it fails. A filter that does not seal in its rack lets unfiltered air slip around the edges, so the rating on the box means nothing because the air is going around it. And a filter at too low a MERV for the job passes the fine particulate it was supposed to catch.

On the walkthrough, pull a filter and look at it. A gray, packed filter is overdue. Daylight around the frame, a hardened gasket, or a filter rattling loose in an oversized rack is bypass, which is a fit problem rather than a filter problem. A throwaway panel where the application needs fine-particle capture is the wrong filter for an IAQ complaint about dust or smoke.

The fix is matching the filter to the job and the fan, sealing it against bypass, and changing it on its measured pressure drop rather than a calendar guess. A higher MERV is not a cure for every complaint, and dropping one into a system with no static headroom starves the airflow and makes the building worse. The whole filter selection, sealing, and change-out question is the Anvilfield air filtration and MERV guide. The investigation's job is to spot the loaded, bypassed, or undersized filter and route it to that fix.

Sick building syndrome versus building-related illness

These two terms get used loosely, and the distinction matters for what you do next. Sick building syndrome describes a pattern where occupants have acute comfort and health complaints, headaches, irritation, fatigue, stuffiness, that are linked to time spent in the building but cannot be tied to a specific illness or a single identified cause, and that ease after people leave. The EPA's own definition turns on exactly that: real effects, no specific illness or cause pinned down. It is a description of a pattern, not a diagnosis.

Building-related illness is the opposite: a specific, diagnosable condition with an identifiable cause traceable to the building. Legionnaires' disease from a contaminated water system is the textbook case, along with hypersensitivity pneumonitis and carbon monoxide poisoning. These are clinical diagnoses made by medical professionals, not by the person holding the meter.

The line an investigator does not cross is medical diagnosis. You can document the building conditions, the complaint pattern, and the measurements. You cannot tell an occupant what is wrong with them, and you should not try. When complaints point toward a specific illness, especially anything in the Legionnaires' or severe-respiratory direction, that goes to medical professionals and an industrial hygienist immediately. Your report describes the building, hedged to what you measured, and refers the health question to the people qualified to answer it.

When to take laboratory samples

Most IAQ investigations are solved by the walkthrough and the screening instruments, without ever sending a sample to a lab. Laboratory sampling earns its place when you have a specific question that the screening cannot answer, and it should be targeted at that question, not sprayed across the building hoping something turns up.

Targeted sampling means you already have a hypothesis. A suspected mold reservoir behind a wall might justify a tape-lift or bulk sample of the suspect material, or a spore-trap comparison of the complaint area against an outdoor baseline, run by someone who can interpret it. A specific chemical concern justifies a sorbent tube or a summa canister sent for compound-specific analysis, not a TVOC screen. A Legionella concern is a water-system sampling question for a qualified lab and protocol. In each case the sample answers a question you framed first.

The shotgun approach, broad air sampling with no hypothesis, is the trap. It is expensive, the results are hard to interpret without a baseline, and outdoor and indoor backgrounds vary so much that a number alone rarely proves anything. This is where an industrial hygienist sets the sampling plan, because the value is in the design of the sampling and the interpretation, not in the act of collecting air. When the question is worth a lab, it is usually worth an IH.

Fix the cause, not the symptom

The point of the whole investigation is to find the cause and correct it, and the cause is almost always ventilation, a source, or moisture. Fix the right one and the complaint goes away and stays away. The temptation, especially when an owner wants something done today, is to put a box in the room and call it handled. That treats the symptom and leaves the cause running.

Sort the fix to the cause you found. Stuffiness and high CO2 mean more outdoor air or a damper and controls correction, the ventilation fix. An odor or off-gassing means removing or sealing the source and airing the space, the source-control fix. A musty smell and visible growth mean finding and stopping the water and removing the wet material, the moisture fix. Dust and fine particulate mean the right filter, sealed, changed on its pressure drop. Each complaint maps to a system, and the system is what you correct.

Source control is the cheapest and most permanent lever, because a contaminant you stop at the source is one the ventilation and filtration never have to chase. Ventilation is next, and it is the answer to the most common complaint of all. An air cleaner is none of these. It is a supplement, covered next, and selling it as the answer to a ventilation or moisture problem is the move that brings the same complaint back next season.

Where air cleaners fit

Portable air cleaners and in-duct add-ons have a place, and that place is a supplement to ventilation and source control, not a replacement for either. A good HEPA-grade portable unit genuinely removes fine particulate from the air in a room, which helps with smoke, allergens, and the particle side of a problem. What it does not do is bring in fresh air, lower CO2, fix moisture, or remove most gases, so it cannot solve the most common causes an investigation turns up.

Be skeptical of the active gadgets. Ionizers, ozone generators, and some so-called air purifiers can produce ozone or other byproducts, and ozone is itself a respiratory irritant, so a device sold to clean the air can add a contaminant. Where an air cleaner is warranted, a mechanical particle filter, the HEPA-style kind, is the predictable choice, and the gas-phase and filtration details are the Anvilfield air filtration and MERV guide.

The honest framing for an owner is that an air cleaner is a layer on top of a building that is already ventilated, dry, and source-controlled. It is real help for the particle fraction in a specific room. It is not a fix for a closed outdoor air damper, a roof leak, or a new-carpet off-gassing problem, and presenting it as one is how a building keeps its complaint and loses trust in the person who recommended the box.

When to bring in an industrial hygienist

Know the edge of your scope and call an industrial hygienist when the investigation crosses it. An IH, or an indoor environmental professional, is trained in ventilation, environmental health, toxicology, and microbiology, which is exactly the mix a complicated IAQ case needs, and the AIHA is the professional body behind that discipline. Bring one in when health complaints are significant or persistent, when a specific contaminant or exposure has to be quantified and defended, when mold or a water-system pathogen is at issue, or when the situation is heading toward a claim or litigation.

The reason to escalate is not just expertise, it is the line around medical and exposure judgments. An IH designs a defensible sampling plan, interprets the results against occupational and reference standards, and frames the findings in a way that holds up. The HVAC investigator finds and fixes the building condition. The IH handles the exposure and health-risk question, and a physician handles the diagnosis. Those are three jobs, and trouble starts when one person tries to do all three.

Do not diagnose. You can and should report what you measured, what you found in the building, and what you corrected. You should not tell an occupant what illness they have or guarantee that the building is or is not making them sick. Hedge the health question to the qualified professional, refer it early, and keep your own work to the building. That restraint is itself a mark of the experienced investigator.

What to document

An IAQ investigation that nobody wrote down is an investigation that cannot be defended when the complaint comes back or a claim is filed. The record is the whole value six months later: it shows what was reported, what you found, what you measured, what the cause was, and what got corrected.

Capture the complaint as taken, who, what, where, and when, the building history you turned up, the walkthrough findings, the instrument readings with the time and location and the outdoor baseline alongside them, the cause you identified, the corrective action, and the follow-up that confirmed the fix held. Note where you referred a health or exposure question to an industrial hygienist or a medical professional, because that referral is part of staying inside your scope. Logging it in the field as you go, with a tool like FieldOS, builds the record from the actual walkthrough and readings instead of a reconstruction from memory back at the office.

The corrective action and the confirmation matter as much as the findings. A report that reads high CO2, opened the outdoor air damper, reread at 750 ppm the next day closes the loop. One that lists readings and stops leaves the next person wondering whether anything was ever fixed.

Field checklist

Common mistakes

• Grabbing a meter and taking numbers before understanding the complaint and the building.
• Missing carbon monoxide by treating it as one reading on the list instead of the first safety check.
• Blaming mold without finding and fixing the moisture source that is feeding it.
• Ignoring a closed or stuck outdoor air damper while the fans move plenty of recirculated air.
• Over-reading a cheap TVOC or PM sensor and putting a screening index in a report as a fact.
• Recommending an air purifier instead of fixing the ventilation, the source, or the moisture.
• Broad air sampling with no hypothesis, producing numbers nobody can interpret.
• Diagnosing an occupant's illness instead of documenting the building and referring the health question.

Standards and references

ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, is the standard behind the outdoor air rates that most ventilation-cause complaints turn on, and ASHRAE Standard 55 governs the temperature and humidity comfort zone that a stuffiness complaint often lives in. Both are republished on a cycle and adopted by jurisdiction, so design and cite to the adopted edition and confirm local amendments before treating any rate, setpoint, or comfort band as fixed.

The EPA is the practical reference for the investigation method itself. Its Building Air Quality guide for owners and facility managers, and its broader IAQ guidance, lay out the complaint-first, walkthrough-driven approach and the moisture-and-ventilation emphasis this guide follows. OSHA does not have a single comprehensive IAQ standard, but it enforces specific limits that apply, including the 50 ppm 8-hour permissible exposure limit for carbon monoxide, and the General Duty Clause where a recognized hazard exists. The AIHA is the professional body for industrial hygiene, and a qualified industrial hygienist or indoor environmental professional is who you bring in for exposure, sampling, and health-risk questions.

None of these makes the investigator a physician. Hedge the levels and the health questions to the adopted ASHRAE editions, the EPA and OSHA guidance, and the industrial hygienist, and keep the medical diagnosis with the medical professionals. Cite the standard that governs the point, confirm the edition, and let the qualified professional own the exposure and health call.

Units, terms, and conversions

IAQ work borrows units and terms from ventilation, comfort, and occupational health, so the same idea can read differently across a complaint form, a controls screen, and a lab report.

Gas concentrations are in parts per million, ppm, for CO2 and CO. Particulate is in micrograms per cubic meter, written µg/m3, and sorted by size in microns, the micrometer. Temperature is in degrees Fahrenheit or Celsius and relative humidity in percent. TVOC is reported as a total in ppb or µg/m3, or as a unitless index on a screening sensor. Air change rate, when it comes up, is in air changes per hour, ACH, the supplied volume divided by the room volume each hour.

IAQ investigation

A structured search for the cause of an air complaint, working from the people and the building to a cause before testing

CO2 as ventilation indicator

Indoor carbon dioxide above the outdoor level, a marker of outdoor air per person, not a toxin at normal indoor levels

Carbon monoxide (CO)

A colorless, odorless, poisonous combustion gas, the deadly IAQ contaminant, checked first

VOCs / TVOC

Volatile organic compounds that off-gas from materials; TVOC is their lumped screening sum, not a single compound

Sick building syndrome

A pattern of acute comfort and health complaints tied to a building with no specific illness or cause identified

Building-related illness

A specific, diagnosable condition with an identifiable building cause, such as Legionnaires' disease

Outdoor-air damper

The damper that admits outdoor air for ventilation; stuck or closed, it is the most common HVAC IAQ fault

PM2.5

Fine particulate up to 2.5 microns, including smoke and combustion soot, the fraction that penetrates deepest

Industrial hygienist

A professional trained in environmental health and exposure, brought in for health, sampling, and litigation questions

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FAQ

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