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Good indoor air quality" gets mentioned constantly in building design and HVAC marketing, but it's rarely defined precisely. It isn't a single measurement, and it isn't fully solved by any one piece of equipment. Good IAQ is the outcome of several distinct factors working together, ventilation, filtration, humidity control, and source control, and understanding what each one actually contributes is what separates a genuinely healthy space from one that simply feels comfortable while quietly falling short.
Ventilation: Bringing in Fresh Air, Deliberately
Ventilation is the deliberate introduction of outdoor air into a building, diluting indoor pollutants, replenishing oxygen, and removing contaminants that accumulate from occupants, materials, and activities inside the space. This is distinct from infiltration, the uncontrolled air leakage covered in envelope design, because ventilation is intentional and measurable, while infiltration is incidental and largely uncontrolled.
Ventilation requirements are typically expressed as a rate, a volume of outdoor air per person or per unit of floor area, per unit of time. These rates aren't arbitrary. They're based on research into how quickly indoor pollutant concentrations, carbon dioxide being a common proxy measurement, build up relative to occupancy and space use, and how much outdoor air dilution is needed to keep those concentrations within an acceptable range.
Under ventilating a space allows pollutants, moisture, and carbon dioxide to accumulate, which can contribute to poor concentration, fatigue, and a general sense that a room feels "stuffy," even when temperature is perfectly controlled. Over ventilating, on the other hand, wastes energy, since outdoor air usually needs to be conditioned before it's introduced, and in humid climates can introduce more moisture load than a system is prepared to handle. Getting ventilation rate right is a genuine balancing act, not simply a matter of maximizing outdoor air.
Filtration: Removing What's Already in the Air
While ventilation dilutes indoor air with outdoor air, filtration removes particulate matter from air as it passes through the HVAC system, whether that air is outdoor air being brought in or indoor air being recirculated.
Filter performance is commonly rated using MERV, Minimum Efficiency Reporting Value, a standardized scale that measures how effectively a filter captures particles of different sizes. Lower MERV ratings capture larger particles, dust, pollen, but let smaller particles pass through relatively freely. Higher MERV ratings capture progressively smaller particles, including many that contribute to respiratory irritation and are too small to see, but at a cost.
That cost is airflow resistance. A higher MERV filter is physically denser, which restricts airflow more than a lower rated filter, requiring the system's fan to work harder to move the same volume of air. Installing a very high MERV filter into a system that wasn't designed to handle that airflow resistance can reduce total airflow, strain the fan motor, and in some cases reduce overall system performance rather than improving air quality, an outcome exactly opposite to the intended goal.
This is why filter selection isn't simply "higher MERV is always better." The right filter rating depends on what the system was actually designed to handle, and matching filtration strategy to the specific HVAC equipment in place is a genuine engineering decision, not just a purchasing one.
Humidity: The Factor That's Easy to Overlook
Relative humidity affects indoor air quality in ways that go beyond simple comfort. Excessively high humidity creates conditions favorable to mold and dust mite growth, both genuine air quality concerns, while excessively low humidity can irritate respiratory passages and increase susceptibility to certain airborne illnesses.
As covered in the distinction between sensible and latent heat, humidity control is a genuinely separate function from temperature control, one that a system only accomplishes properly with sufficient runtime and appropriately sized latent capacity, not simply by satisfying a thermostat setpoint. A space that reads at a comfortable temperature can still present real IAQ concerns if humidity is poorly controlled, whether too high or too low.
Source Control: The Factor Ventilation and Filtration Can't Fully Compensate For
Ventilation and filtration both work to dilute or remove contaminants after they've entered the air. Source control addresses the problem differently, by reducing or eliminating pollutants at their origin, low emission building materials, proper storage of chemicals and cleaning products, adequate exhaust at specific pollutant sources like kitchens or printing areas, rather than relying entirely on dilution and filtration to manage whatever gets released into the space.
This matters because ventilation and filtration both have practical limits. No ventilation rate is unlimited, and no filter captures everything. A space with poor source control can genuinely overwhelm reasonable ventilation and filtration capacity, meaning good IAQ strategy has to consider what's being introduced into a space, not only how well the HVAC system dilutes and filters it afterward.
What "Good IAQ" Actually Means
Given all of this, good indoor air quality isn't a single number or a single piece of equipment. It's the combined result of ventilation rate appropriately matched to occupancy and space use, filtration appropriately matched to the system's actual airflow capacity, humidity genuinely controlled rather than incidentally managed as a byproduct of temperature control, and source control reducing the pollutant load the other three factors have to handle in the first place.
A space can fail on any one of these factors while appearing entirely fine on the others, comfortable temperature and adequate filtration but poor ventilation, for instance, and still present a genuine air quality problem. Treating IAQ as a single checkbox, rather than the combined outcome of several genuinely distinct factors, is a common gap between a design that looks complete on paper and a space that's actually healthy to occupy.
EveryEng. For everyone in Engineering.
Aug 29, 2026 
