The Science Behind Indoor Air Quality>>>

By INDEX Editorial Team | Based on peer-reviewed research>>>

Most people think of “air pollution” as something that lives outside—smog, highway exhaust, industrial stacks. Yet Americans spend about 90% of their time indoors, and multiple studies from the U.S. Environmental Protection Agency (EPA) and National Institute of Environmental Health Sciences (NIEHS) have found that indoor air can be 2–5 times, and sometimes up to 100 times, more polluted than outdoor air in the same area.

Indoor air quality (IAQ) matters because:

• Pollutants are often more concentrated indoors.
• Buildings can trap contaminants instead of dispersing them.
• We breathe indoor air for long, continuous stretches—at home, at work, in schools and healthcare facilities.

This article walks through the science behind indoor air quality in a clear, practical way: what the main pollutants are, how they behave, what the evidence says about health, and what criteria to use when you decide how to test and improve the air in your own spaces.

INDEX (Indoor Exposure Index) is a 501(c)(3) environmental health nonprofit. Our role is similar to a Consumer Reports–style organization for indoor environments: we do not have a financial incentive to favor one brand over another. Our mission incentive is to reduce harmful exposure indoors using a science‑to‑solution (S2S) approach.

How Scientists Define Indoor Air Quality

Indoor air quality refers to the chemical, physical, and biological characteristics of air inside buildings and how those characteristics affect occupant health, comfort, and performance.

Research and standards organizations such as EPA, NIEHS, and ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers) focus on three core dimensions:

1. Pollutant Sources – What’s being released into the air?
2. Transport and Transformation – How pollutants move, dilute, react, or settle.
3. Exposure and Health Outcomes – How much people actually inhale over time, and what that does to the body.

When you see an indoor air “score,” a monitor reading, or a building rating, it is usually some combination of these factors distilled into a simpler metric.

The Main Categories of Indoor Air Pollutants

Indoor pollutants fall into a few broad scientific categories. Understanding these is the first step in making sense of test results, product claims, or building strategies.

1. Particulate Matter (PM2.5, PM10, and Ultrafine Particles)

What it is:
Tiny solid and liquid particles suspended in air. Common size ranges:

• PM10: Particles ≤10 microns (µm) in diameter
• PM2.5: Particles ≤2.5 µm (fine particles that penetrate deep in lungs)
• Ultrafine particles: <0.1 µm (not yet widely regulated, but actively studied)

Sources indoors:

• Cooking (especially frying, grilling, toasting)
• Candles, incense, fireplaces, and wood stoves
• Tobacco smoke and vaping aerosols
• Outdoor pollution infiltrating indoors (traffic, wildfire smoke, industrial sources)
• Some cleaning methods (e.g., aggressive dusting, vacuuming without effective filtration)

Why it matters:
Fine and ultrafine particles can reach deep into the lungs and, in some cases, enter the bloodstream. EPA and NIEHS link elevated PM2.5 exposure to:

• Asthma symptoms and COPD exacerbations
• Cardiovascular effects (heart attacks, arrhythmias, hypertension)
• Increased risk of premature death in people with heart or lung disease

Indoors, short spikes (like during cooking) can reach very high levels, even if outdoor air is relatively clean.

2. Gases and Volatile Organic Compounds (VOCs)

What they are:
Gases include carbon monoxide (CO), carbon dioxide (CO₂), nitrogen dioxide (NO₂), ozone (O₃), and others. Volatile organic compounds (VOCs) are carbon-based chemicals that readily become vapors at room temperature.

Common indoor sources:

• Gas stoves and unvented combustion appliances → CO, NO₂
• Building materials, paints, adhesives, new furniture → formaldehyde and other VOCs
• Cleaning products, air fresheners, fragrances → many VOCs, some known irritants
• Office equipment (printers, copiers) → ozone and VOCs
• Outdoor pollution seeping indoors → ozone, NO₂, VOCs

Why they matter:

• Carbon monoxide (CO): Colorless, odorless, toxic gas. High levels can be fatal; even moderate levels can cause headaches, dizziness, and confusion.
• Nitrogen dioxide (NO₂): Irritates airways, associated with asthma symptoms and respiratory infections, particularly in children.
• Formaldehyde and other VOCs: Short-term effects include eye, nose, and throat irritation, headaches, and nausea. Long-term exposure to some VOCs has been linked to cancer and organ toxicity.
• CO₂: Primarily a ventilation indicator. Elevated indoor CO₂ often signals that people are rebreathing each other’s exhaled air and that other pollutants may also be accumulating. Multiple studies suggest that high indoor CO₂ is associated with reduced cognitive performance and decision-making.

3. Biological Contaminants (Mold, Bacteria, Viruses, Allergens)

What they are:

• Mold spores and fragments
• Bacteria and their byproducts (endotoxins)
• Viruses (including those responsible for COVID-19, influenza, RSV)
• Allergens (dust mite particles, pet dander, cockroach allergens, pollen that infiltrates indoors)

Sources and conditions:

• Moisture problems: leaks, condensation, high humidity → mold growth
• Poor ventilation and high occupancy → higher viral and bacterial load in shared air
• Soft furnishings and clutter → reservoirs for dust and allergens

Why they matter:

• Mold and dampness are associated in the scientific literature with respiratory symptoms, asthma development and exacerbation, and some immune effects.
• Airborne transmission of respiratory viruses is now well documented; ventilation and filtration affect how far and how long infectious aerosols linger.
• Allergens can trigger asthma attacks and allergic rhinitis, especially in children and sensitized adults.

4. Radon and Other Less Visible Hazards

Radon is a radioactive gas that seeps from soil into buildings, especially basements and ground floors. It is colorless and odorless, but long-term exposure is the second leading cause of lung cancer in the United States after smoking, according to EPA.

Other niche hazards (e.g., asbestos fibers, lead dust) are important in specific building types and ages but are less commonly discussed under general “IAQ” unless there is renovation or damage.

How Indoor Pollutants Behave: The Basic Physics and Chemistry

Understanding a few simple mechanisms helps explain why some fixes work—and why others may disappoint.

Source → Transport → Removal

Every pollutant indoors follows a basic path:

1. Source: Emitted from a surface, activity, or occupant (e.g., cooking, cleaning, breathing).
2. Transport: Diluted or concentrated depending on room size, air mixing, and airflow patterns.
3. Removal or Transformation:
   • Ventilation replaces indoor air with outdoor air.
   • Filtration removes particles from circulating air.
   • Deposition causes particles to settle on surfaces.
   • Chemical reactions convert gases into other compounds (e.g., ozone reacting with terpenes in scented cleaners to form secondary pollutants).

The Role of Ventilation

ASHRAE standards (such as ASHRAE 62.1 and the 2023 ASHRAE guideline on enhanced IAQ) emphasize ventilation rate and air distribution. Key points:

• Higher ventilation (within reason) generally reduces the concentration of many indoor-generated pollutants.
• Poorly designed or maintained systems can short-circuit air movement, leaving pockets of stagnant, polluted air even when the total airflow looks adequate on paper.
• Natural ventilation (open windows) can work well in some climates and seasons but may introduce outdoor pollution or humidity issues in others; as well as security concerns.

The Role of Filtration

Mechanical filtration targets particulate matter, not gases:

• Filters are rated by MERV (Minimum Efficiency Reporting Value). Higher MERV (e.g., 13–16) generally captures finer particles more effectively.
• Portable air cleaners often use HEPA (High Efficiency Particulate Air) filters designed to capture at least 99.97% of particles of 0.3 µm under test conditions.
• Gases and VOCs require different media (e.g., activated carbon, specific sorbents).

Health Effects: What the Evidence Shows

The health science behind indoor air quality covers a spectrum from immediate irritation to long-term chronic disease. Major research bodies, including EPA, NIEHS, the World Health Organization, and peer-reviewed journals, highlight several outcome categories.

Short-Term (Acute) Effects

These can appear within minutes to hours of exposure:

• Eye, nose, and throat irritation
• Coughing, wheezing, or shortness of breath
• Headaches, fatigue, and difficulty concentrating
• Dizziness or nausea in more severe cases (particularly with CO or strong VOC exposures)

The term “sick building syndrome” has often been used when occupants report these symptoms in a specific building without a single clear cause—but modern research tends to focus on specific pollutants, ventilation metrics, and moisture problems rather than the catch-all label.

Long-Term (Chronic) Effects

Chronic exposure to certain indoor pollutants is associated with:

• Asthma onset and worsening, particularly in children living in damp or moldy homes
• Chronic obstructive pulmonary disease (COPD) exacerbations
• Cardiovascular disease linked to PM2.5 exposure
• Lung cancer risk from radon and, in some settings, from long-term secondhand smoke exposure
• Emerging evidence of cognitive and developmental impacts from sustained exposure to certain pollutants (e.g., PM2.5, NO₂, and some VOCs)

Vulnerable Populations

Some groups are more affected at a given exposure level:

• Children (developing lungs and immune systems, higher breathing rates per body weight)
• Older adults
• People with asthma, COPD, or cardiovascular disease
• Pregnant individuals and fetuses
• Workers with high occupational exposure (e.g., cleaning staff, industrial workers)

A Criteria-First Framework for Evaluating IAQ Solutions

With rising awareness, the marketplace is crowded with monitors, filters, sprays, and “air cleansing” technologies. Instead of focusing on brand names, a criteria-first approach helps separate science-based options from marketing noise.

1. Source Control First

Evidence-based IAQ frameworks (EPA, ASHRAE, and multiple reviews in the scientific literature) consistently prioritize source control:

• Combustion:
  • Use properly vented range hoods over gas stoves.
  • Avoid unvented combustion heaters indoors.
  • Prohibit smoking indoors.
• VOCs and Fragrances:
  • Favor low-VOC paints, adhesives, and furnishings where feasible.
  • Limit use of air fresheners and heavily fragranced products, especially in small or poorly ventilated rooms.
• Moisture and Mold:
  • Aim for indoor relative humidity around 30–50%.
  • Address leaks, condensation, and drainage issues quickly.
  • Use local exhaust in bathrooms and kitchens.

Why this criterion matters: Reducing or eliminating a pollutant at its source prevents it from ever entering the air you breathe. It is usually more effective and cost-efficient than trying to clean heavily polluted air after the fact.

2. Ventilation: Enough Fresh Air, Well Distributed

When evaluating or upgrading ventilation:

• Fresh air provision: Does the space meet or exceed relevant ASHRAE or local building code ventilation rates?
• Airflow patterns: Are supplies and returns arranged to avoid dead zones where air is stagnant?
• Demand-controlled ventilation: In some buildings, CO₂-based ventilation control can increase fresh air when occupancy is high.

Why this criterion matters: Ventilation dilutes indoor-generated pollutants and is central to controlling CO₂ buildup and infectious aerosol concentrations in shared spaces.

3. Filtration and Air Cleaning: Match Technology to Pollutant

When considering filtration:

• Particle removal: For central HVAC systems, filters in the MERV 13–16 range (if compatible with the system) are often cited in expert guidance for reducing fine particles and some infectious aerosols.
• Portable air cleaners: Clean Air Delivery Rate (CADR), appropriate sizing for room volume, and filter replacement costs are key metrics.
• Gases and VOCs: Only certain media (e.g., activated carbon) address these; basic particle filters do not.

Why this criterion matters: Different pollutants require different control strategies. A high-end particle filter will not significantly reduce VOCs, and a small portable unit will not effectively treat a large open-plan office.

4. Measurement and Transparency

To make informed decisions, look for:

• Clear performance metrics (e.g., CADR, MERV rating) instead of vague claims like “kills 99% of germs.”
• Independent testing data, ideally from recognized labs or peer-reviewed research.
• Avoidance of unproven or potentially harmful technologies, such as devices that intentionally generate ozone indoors.

Why this criterion matters: Independent data help distinguish well-characterized solutions from those driven primarily by marketing.

Practical Pathways: Applying the Science in Homes and Workplaces

Using the criteria above, here is a simple, science-informed sequence for most homes and many workplaces.

Step 1: Identify Key Sources

Note the main contributors to indoor pollutants:

• Any gas appliances, especially older stoves or heaters.
• Potential VOC sources in each room (new furniture, renovations, strong-smelling products).
• Moisture signs: visible mold, musty odors, condensation on windows, damp basements.
• Occupant density and activities (cooking, cleaning, hobbies that generate dust or fumes).

Step 2: Improve Everyday Habits

Day-to-day practices can significantly change indoor exposure:

• Use exhaust fans when cooking and showering; verify they actually exhaust to outdoors.
• Avoid burning candles or incense frequently, especially in small rooms.
• Store solvents, paints, and harsh chemicals in well-ventilated or detached areas if possible.
• Choose fragrance-free or low-VOC cleaning and personal care products where feasible.

Step 3: Address Ventilation and Filtration

Strengthen the building systems that control indoor air:

• For homes with HVAC:
  • Use filters in the highest MERV rating your system can safely handle.
  • Replace filters on schedule.
• For apartments, schools, or offices:
  • Ask building management about filtration level and outdoor air rates.
  • In spaces without mechanical ventilation, consider window ventilation when outdoor conditions are acceptable, while being mindful of outdoor pollution or humidity, and security issues.

Step 4: Consider Targeted Air Cleaning

Use room-level devices where they can do the most good:

• In bedrooms or high-use living areas, a correctly sized portable air cleaner with a high-efficiency particle filter can reduce particle exposure, especially during wildfires or heavy cooking.
• In offices, conference rooms, and classrooms, portable units can supplement central systems during high occupancy or infection surges.

Step 5: Test Strategically

Testing can help you prioritize and verify improvements:

• Radon: EPA and public health agencies recommend radon testing in many areas, especially for ground-contact spaces.
• CO and smoke alarms: Ensure functioning carbon monoxide and smoke detectors where combustion appliances exist.
• IAQ monitors: Devices that display PM2.5, CO₂, and sometimes VOC indices can provide helpful trend information. While consumer monitors vary in accuracy, they can reveal patterns (e.g., cooking spikes, consistently high CO₂ in a meeting room).

Quantify Your Own Indoor Air Risk

To move from general science to your specific situation, INDEX provides a free, research-informed tool:

Try the IAQ Risk Calculator
– Answer a short set of questions about your building, activities, and occupants.
– Receive a risk score plus targeted suggestions aligned with the criteria above.
– Get an optional summary you can share with family members, building management, or your safety committee.

Access it at: IAQ Risk Calculator

To view your full results and download a personalized checklist, you’ll be asked to enter an email address. This helps us deliver your report and share updates as new research emerges.

Cleaning Products and Indoor Air Quality

While INDEX does not endorse specific brands, cleaning chemistry is one area where product choice can significantly influence indoor air.

Criteria Framework: What to Look For in a Less-Toxic All-Purpose Cleaner

From an indoor air perspective, key criteria include:

1. No or very low VOCs – Reduces chemical fumes that can irritate eyes and lungs.
2. No synthetic fragrances or dyes – Many fragrances are VOC mixtures; dyes are unnecessary for cleaning performance.
3. No known respiratory irritants (e.g., 2-butoxyethanol) – This solvent is common in degreasers and can penetrate skin and irritate lungs.
4. No PFAS (“forever chemicals”) – Persistent compounds that may have long-term health and environmental impacts.
5. Biodegradable surfactants – Break down relatively quickly in the environment.
6. Plant-derived or otherwise well-characterized ingredients – With clear safety data for regular indoor use.
7. pH-neutral and free of harsh acids, ammonia, or bleach – Especially in spaces without strong ventilation.
8. Food-contact safety where relevant – For surfaces where food is prepared or children play.

Why These Criteria Matter for IAQ

Cleaning products can be an important source of VOCs and secondary pollutants indoors. Studies have shown that certain scented cleaners and air fresheners emit terpenes and other VOCs that can react with ozone to form ultrafine particles and formaldehyde. Products meeting the criteria above can help reduce:

• Irritating fumes during and after cleaning
• Chemical residues that become part of household dust
• Secondary pollutant formation from indoor chemical reactions

A Practical Option That Meets These Criteria

Based on independent information available and the criteria listed above, one practical option for consideration is:

Red Juice by Speed Cleaning
– USDA A-1 rated for use on food-contact surfaces
– No PFAS, no VOCs, and no synthetic fragrances or dyes
– Biodegradable, plant-derived formulation (soy- and seaweed-based surfactants)
– No 2-butoxyethanol, ammonia, bleach, or harsh acids; pH-neutral

For readers who wish to explore this product further, it is available here: Speed Cleaning

How INDEX Uses Science to Inform Indoor Air Decisions

The INDEX Science Center (at S2S) tracks peer-reviewed research projects on indoor exposures, including particulate matter, VOCs, and building-related health outcomes. Our science-to-solution (S2S) methodology follows three steps:

1. Map the evidence – Identify which exposures are most strongly linked to harm.
2. Translate to practical criteria – Convert research findings into clear, measurable criteria (like those above for cleaning products and ventilation).
3. Highlight practical pathways – Point to building practices, tools, and products that align with those criteria, without endorsing specific brands.If you find this information useful and want to support more research like this, you can learn about giving options on our site.

Key Takeaways

• Indoor air quality is shaped by a combination of sources, building systems, and everyday activities.
• Major pollutant categories—particles, gases/VOCs, biological contaminants, and radon—have well-documented health impacts across short- and long-term timeframes.
• Evidence-based strategies prioritize source control, adequate ventilation, effective filtration, and transparent performance data.
• Product choices, e.g., in cleaning, can meaningfully influence indoor exposures; criteria-based selection is more reliable than marketing claims.
• Tools like the INDEX IAQ Risk Calculator can translate general science into actionable next steps for your specific home, school, or workplace.

By combining scientific understanding with practical criteria, you can improve indoor air quality in a way that is measurable, sustainable, and aligned with long-term health—not just short-term trends.