Indoor Environmental Quality Research>>>

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

Indoor environmental quality (IEQ) used to be a niche topic. Today it sits at the center of conversations about health, learning, and productivity. We spend roughly 90% of our time indoors, and a growing body of research shows that air, light, sound, and thermal conditions inside buildings can either support or undermine our brains and bodies.

This article summarizes key trends in indoor environmental quality research, explains what they mean for real-world decisions, and outlines practical steps for both homeowners and facility managers.

What Is Indoor Environmental Quality?

Researchers and standards bodies typically define IEQ as the combined effect of several indoor conditions on people in a building. Most frameworks now include at least five dimensions:

1. Air quality – ventilation, particulate matter, gases (e.g., CO₂, VOCs), biologicals (mold, allergens).
2. Thermal comfort – temperature, humidity, air movement.
3. Lighting – intensity, spectrum, timing, glare, daylight access.
4. Acoustics – background noise, reverberation, speech privacy.
5. Other factors – such as odors or crowding, depending on the study.

Recent systematic reviews (2023–2025) emphasize that these factors interact. For example, improving energy efficiency by tightening a building envelope without adjusting ventilation can worsen air quality and CO₂ levels, which then affect cognition and comfort.

Why IEQ Research Matters Now

Three trends are driving intensive research into IEQ:

Health and vulnerable populations
Reviews focused on older adults and vulnerable groups (e.g., those with asthma or cardiovascular disease) show that poor indoor environments increase symptoms and healthcare use, particularly in residential settings and long-term care facilities.

Brain function and productivity
A 2026 systematic review on IEQ and the brain highlighted associations between indoor conditions and cognitive performance, mood, and sleep. Elevated CO₂, fine particles, and suboptimal lighting were repeatedly linked to slower decision-making and more fatigue in office and school settings.

Energy and climate policy
Papers on “balancing indoor environmental quality and energy efficiency” show that decarbonization and retrofit efforts can unintentionally degrade IEQ if ventilation, filtration, and moisture control are not addressed at the same time.

For building owners, facility managers, and households, the question is no longer whether IEQ matters, but how to prioritize improvements based on the best available evidence.

How Researchers Measure IEQ

Different research groups use different tools, but common approaches include:

Objective measurements
– CO₂ (as a proxy for ventilation effectiveness)
– Particulate matter (PM₂.₅, PM₁₀)
– Temperature and relative humidity
– Volatile organic compounds (VOCs)
– Illuminance (lux) and sometimes spectral power distribution of light
– Sound pressure levels (dBA)

Standards and guidelines as reference points
– ASHRAE 62.1 / 62.2 – ventilation and acceptable indoor air quality in commercial and residential buildings.
– ASHRAE 55 – thermal environmental conditions for human occupancy.
– Emerging IEQ frameworks that integrate multiple dimensions (e.g., draft ASHRAE IEQ standards, healthy building certification systems).

Occupant-centered data
– Surveys on comfort, perceived air quality, noise annoyance, and lighting satisfaction.
– Symptom checklists related to “sick building syndrome.”
– Task performance tests (e.g., decision-making speed, error rates) in office and school studies.

The consistent message across studies: objective measurements and occupant reports both matter. An environment can meet minimum standards yet still feel uncomfortable or distracting to occupants.

Key Findings by IEQ Dimension

1. Air Quality: Ventilation, Pollutants, and Health

The U.S. EPA and multiple reviews highlight several recurring themes:

Ventilation rates and CO₂
– ASHRAE guidance for homes (e.g., Standard 62.2) suggests mechanical ventilation rates around 0.35 air changes per hour and not less than 15 cfm per person.
– Practical rule of thumb in many studies: keep indoor CO₂ below about 1,000 ppm and ideally no more than 700 ppm above outdoors. Higher levels have been associated with reduced decision-making performance and increased complaints of drowsiness.

Particulate matter (PM₂.₅)
– Studies repeatedly associate higher indoor PM₂.₅ with respiratory and cardiovascular risks, especially for children and older adults.
– Typical “good” targets used in research and health guidance: below ~12 µg/m³ for PM₂.₅ over 24 hours, aligning with common health-based benchmarks.

Moisture, mold, and dampness
– Research on residential buildings and schools shows consistent links between visible mold, dampness, and increased respiratory symptoms, particularly asthma exacerbations in children.
– Effective moisture control (fixing leaks, managing humidity, avoiding cold surfaces where condensation forms) is a recurring protective factor.

Chemical pollutants (VOCs, cleaning agents)
– Indoor sources—paints, furnishings, cleaning products—can emit VOCs that irritate airways and may have longer-term health implications.
– Recent work stresses that “green” labels are not always aligned with health-centered criteria; ingredient transparency and absence of specific chemicals (e.g., certain glycol ethers, PFAS) are increasingly emphasized.

Practical implications
– For facility managers: measure CO₂ and PM where possible, check that ventilation and filtration align with current ASHRAE guidance, and integrate moisture inspections into routine maintenance.
– For households: prioritize source control (e.g., low-emission materials and cleaners), use mechanical exhaust in kitchens and bathrooms, and manage humidity in the 40–60% range where feasible.

2. Thermal Comfort: More Than Just Temperature

ASHRAE 55 and related research show thermal comfort is multi-factorial:

Temperature ranges
– Many institutional guidelines adopt approximate comfort bands such as:
– Winter: ~68–76°F (20–24°C)
– Summer: ~72–80°F (22–27°C)
– Individual preferences vary, and acclimatization matters.

Relative humidity
– Several building health reviews and position documents converge on 30–60% relative humidity as a practical target, with 40–60% often cited as a sweet spot:
– Too low: dry eyes, skin, and mucous membranes; more airborne viral persistence.
– Too high: increased dust mite and mold growth, and more off-gassing from some materials.

Productivity and symptoms
– Studies in offices and educational buildings show that even modest deviations from preferred temperature ranges can decrease self-reported productivity and increase complaints (headaches, fatigue).

Practical implications
– For both homes and workplaces, thermal comfort should be assessed by combining:
– Measured temperature and humidity.
– Occupant feedback, especially from those at higher risk (older adults, people with chronic conditions).

3. Lighting: Visual Comfort, Circadian Rhythms, and Performance

Recent IEQ research recognizes lighting as both a visual and biological input:

Visual aspects
– Adequate illumination (lux levels matched to tasks) and low glare are fundamental.
– High color-rendering light sources improve visual clarity and reduce eye strain for detailed tasks.

Biological and circadian aspects
– Studies show that bright, blue-enriched light in the earlier part of the day can support alertness and circadian alignment, while dimmer, warmer light in the evening supports melatonin production and sleep.
– Research on daytime indoor light exposure suggests many people experience “indoor dimness” that is far below outdoor daylight levels, potentially contributing to fatigue and mood issues.

IEQ and the brain
– Reviews focusing on IEQ and cognitive performance report that lighting quality interacts with air quality; suboptimal combinations (e.g., low light plus elevated CO₂) are associated with slower task performance and more errors.

Practical implications
– Seek a daily pattern that approximates “nutritional light”:
– Brighter, fuller-spectrum light during daytime work hours.
– Reduced intensity and blue content in the evening.
– For offices and schools, integrate daylight where feasible while controlling glare, and ensure task lighting is adequate for reading and screen work.

4. Acoustics: Noise, Distraction, and Well-Being

While historically less studied than air and temperature, noise is an increasing focus:

Noise and cognitive load
– Office studies show that elevated background noise and poor speech privacy increase distraction, error rates, and perceived stress.
– Open-plan offices without acoustic treatments often produce more complaints than enclosed spaces with controlled reverberation.

Schools and vulnerable groups
– Research in educational settings indicates that higher background noise and reverberation are associated with poorer reading and comprehension in children, particularly those with hearing or learning differences.

Practical implications
– Consider acoustics as a full IEQ dimension:
– Use acoustic ceiling tiles, carpets, and partitions where appropriate.
– Manage mechanical system noise and consider sound masking systems thoughtfully, verifying that they reduce intelligible distractions rather than adding annoyance.

How IEQ Research Is Used in Real Buildings

Systematic reviews and field studies point toward several cross-cutting themes:

1. Integrated design and operation
Buildings that perform well on IEQ typically address ventilation, filtration, moisture, lighting, and acoustics together, rather than in isolation.

2. Post-occupancy evaluation (POE)
Many higher-performing buildings use occupant surveys and targeted measurements (CO₂, temperature, noise, light levels) to verify that design intentions align with real-world conditions.

3. Balancing IEQ and energy
Research on retrofits emphasizes:
– Tightening envelopes without upgrading ventilation and moisture control can worsen IEQ.
– Smart controls, demand-controlled ventilation, and high-efficiency filtration can help maintain IEQ while managing energy use.

4. Attention to vulnerable populations
Recent reviews highlight the importance of tailoring IEQ strategies for:
– Older adults in residential and long-term care settings.
– Children in schools and childcare environments.
– Individuals with respiratory and cardiovascular conditions.

Criteria Framework: How to Evaluate IEQ in Your Space

Drawing on current research and standards, a practical IEQ evaluation can start with these criteria:

1. Ventilation and CO₂
– Is there mechanical or natural ventilation in main occupied areas?
– Are CO₂ levels typically below 1,000 ppm during occupancy when measured?

2. Particles and Filtration
– Is there effective filtration on central HVAC systems?
– Are there local sources of combustion or heavy particulates (e.g., smoking, frequent frying, unvented heaters)?

3. Moisture and Mold
– Any visible mold, musty odors, or recurrent damp spots?
– Humidity generally in the 40–60% range?

4. Chemicals and Source Control
– Are low-emission materials, furnishings, and cleaning products used where possible?
– Are strong solvents, aerosols, and fragranced products minimized?

5. Thermal Comfort
– Are temperatures generally within broadly accepted comfort ranges?
– Do occupants report being too hot, too cold, or drafty?

6. Lighting Quality
– Is there adequate light for tasks without glare?
– Do occupants have access to brighter, fuller-spectrum light during the day and the ability to dim and warm light in the evening?

7. Acoustics
– Are conversations and equipment noise frequently disruptive?
– Are there materials or layouts that help absorb sound and improve privacy?

Practical Pathways: Applying IEQ Research Step by Step

For Households

Start with the basics
– Use kitchen and bathroom exhaust fans when cooking or showering.
– Address any visible leaks or dampness promptly.
– Avoid smoking indoors and limit high-emission products.

Incremental upgrades
– Add localized task lighting for reading and desk work.
– Use quieter fans and appliances where possible.
– Choose cleaning and maintenance products with transparent, lower-toxicity ingredient profiles.

Assess your overall indoor health profile
– Use structured tools to evaluate your home’s air, light, and comfort conditions and identify the highest-impact changes first.

For Facility Managers and Organizations

Measure, then manage
– Pilot monitoring for CO₂, temperature, and humidity in representative zones.
– Pair measurement data with occupant surveys for a more complete picture.

Align with current standards
– Compare ventilation and thermal conditions with ASHRAE 62.1/62.2 and 55 guidance.
– Integrate IEQ checks into routine preventive maintenance.

Communicate with occupants
– Share what is being monitored and improved.
– Provide channels for occupants to report issues (e.g., noise, drafts, odors) that may not show up on sensors.

Quantify Your Building’s IEQ in Minutes

To translate research into action, INDEX offers a free, research-informed assessment tool:

Try the Healthy Indoor Scorecard
– Answer 25 questions about your air, light, thermal comfort, and noise conditions.
– Receive a simple PDF report highlighting strengths, potential concerns, and practical next steps grounded in current evidence.

Access it at: Healthy Indoor Scorecard

The full report is delivered by email so you can share it with family members, building owners, or decision-makers and track improvements over time.

Product-Aware Segment: Cleaning Products and IEQ

While indoor environmental quality research covers many dimensions, cleaning and maintenance products are a notable source of indoor chemical exposure. Many conventional degreasers and all-purpose cleaners can emit VOCs and contain ingredients that irritate the lungs and skin.

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

Based on available ingredient data and emerging research on indoor exposures, practical criteria include:

1. Food-contact safety rating
– Products rated for use on food-contact surfaces (e.g., USDA A-1) are generally formulated with tighter constraints on residues.

2. No synthetic fragrances or dyes
– Fragrances can be complex mixtures that add VOCs without contributing to cleaning performance.

3. No intentionally added PFAS (“forever chemicals”)
– PFAS are persistent in the environment and are an emerging concern for long-term health.

4. Low or no VOCs
– Aim for formulations that do not rely on high-VOC solvents, especially in poorly ventilated spaces.

5. Readily biodegradable surfactants
– Surfactants that break down quickly reduce environmental load after disposal.

6. Safer surfactant classes
– For example, linear primary alcohol ethoxylates are often considered preferable to some older surfactant chemistries.

7. Plant-derived base where feasible
– Ingredients derived from plant sources (e.g., soy, seaweed) can support biodegradability, although “plant-based” alone is not a guarantee of safety.

8. Absence of specific high-concern ingredients
– No 2-butoxyethanol (a glycol ether associated with respiratory and skin irritation).
– No harsh acids, ammonia, or bleach in general-purpose products used frequently and without specialized PPE.

A Practical Option That Meets These Criteria

A product that aligns with the criteria above and may be useful for consideration in homes and professional cleaning settings is:

Red Juice All-Purpose Degreaser (Speed Cleaning)
– USDA A-1 rated for food-contact surfaces.
– Plant-derived base (soybeans and seaweed).
– No synthetic fragrances or dyes.
– No added PFAS and no VOCs based on available product information.
– Biodegradable, with manufacturer data indicating breakdown within several days.
– Uses linear primary alcohol ethoxylate surfactants rather than more persistent or irritating alternatives.
– Free of 2-butoxyethanol and formulated without harsh acids, ammonia, or bleach; pH-neutral in typical use.

For readers who want to explore this option, more detail is available at: Speed Cleaning.

How INDEX Connects Research to Practice

As a 501(c)(3) environmental health nonprofit, INDEX focuses on translating peer-reviewed research on indoor environments into practical tools and pathways:

Science-to-Solution (S2S) methodology
We map evidence from research (including work cataloged in the INDEX Science Center) to concrete exposure-reduction steps that households and organizations can realistically take.

No financial incentive to favor specific brands
Our trust proposition is straightforward: we do not earn more for highlighting one compliant product over another. When we present products that meet clearly defined criteria, it is to offer a well-informed nod to possible solutions—not an endorsement.

Research and project mapping
The INDEX Science Center (S2S) tracks active research projects and funding needs, including work aimed at better characterizing indoor exposures and their health impacts. Current projects, such as the BEMI initiative, focus on closing data gaps in real-world buildings.

If you find this work valuable and want to help close the evidence-to-action gap in indoor environmental quality, you can support ongoing research and public tools by contributing to INDEX and, in particular, helping fill the remaining funding gap for current Science Center projects.

By grounding decisions in IEQ research and using structured tools to assess your own spaces, you can move from generic advice to targeted, evidence-informed improvements that make a measurable difference in health, comfort, and performance.