By Kristina Held

This post was originally published on HELD Architecture's site.

As we have all heard many times, most of us spend about 90 percent of our lives indoors. The classroom at two in the afternoon, the apartment after a humid night, the office the week after new flooring goes in: those rooms are the air the body has to live in.

Indoor concentrations of many pollutants run higher than outdoor air. Energy-tight construction without planned ventilation, synthetic finishes, crowded rooms, and neglected water systems all add to the load. Architecture can reduce that load. That is the subject of this series — how planning, detailing, and ventilation show up in lungs, cognition, sleep, and immune function.

Moisture and Mold

Mold follows water. Recurring leaks, cold interior surfaces, bathrooms without reliable exhaust, and air-conditioning coils that stay wet give it a place to grow.

In 2025, researchers at UT Southwestern reviewed 231 patients with hypersensitivity pneumonitis, an inflammatory lung disease. Household mold was identified as the antigen for 54 of them. The usual locations were bathrooms, bedrooms, and air-conditioning units, almost always after repeated water damage. Among patients who removed the source — by remediation or by moving — some showed measurable improvement in lung function within a few months, including a few with fibrotic disease.

Other recent work associates visible home mold and dampness with adult asthma and with higher rates of asthma emergency visits.

A smaller group of occupants react more broadly. Clinicians who treat complex environmental illness, including Dr. Jill Carnahan and Dr. Lauren Tessier, describe patients with fatigue, cognitive difficulty, respiratory symptoms, food sensitivities, and inflammatory patterns that can resemble autoimmune flare after time in water-damaged buildings. They often use the term Chronic Inflammatory Response Syndrome. Conventional guidelines, including the Centers for Disease Control and Prevention's materials on mold, do not treat CIRS as an established diagnosis, and the criteria remain contested. The first step is not contested: stop the exposure. A wet building will keep delivering the dose.

Mental health belongs in the same file, with the same care about what the evidence can support. A 2024 review in Environmental Health Perspectives of 19 studies found associations between damp or moldy housing and depression, stress, and anxiety in adults, and emotional symptoms in children. An earlier World Health Organization housing survey of nearly 6,000 European adults also found higher rates of reported depression in damp, moldy homes. Some of that association likely runs through chronic physical illness and the strain of living in a house that feels unsafe. Laboratory studies have shown that inhaled mold spores can activate innate immune responses in the brain and affect memory and behavior in animals.

We do not have to settle every clinical debate before we change how we build. Keep water out of the assembly. Design wall systems that can drain and dry when water does get in. Calculate the dew-point temperature through the section and know where condensation would form. Keep that plane out of moisture-sensitive layers, or give those layers a way to dry to one side or the other. Ventilate wet rooms so they can dry. In the Carolinas, that means rain control, continuous insulation that keeps interior surfaces above dew point, and properly sized mechanical systems.

Volatile Organic Compounds (VOCs)

VOCs are gases given off by paints, adhesives, composite wood, vinyl flooring, sealants, cleaners, and new furniture. Indoors, many of these chemicals are two to ten times higher than outdoors.

Short-term effects include eye and throat irritation, headache, dizziness, and the fog people notice in a newly finished room. Some VOCs are linked to longer-term effects on the liver, kidneys, and nervous system. Formaldehyde remains a particular concern in recently completed interiors. Studies in laboratory houses and newly renovated rooms find more building-related symptoms where VOC levels are high, especially among people with existing allergies or chemical sensitivity.

Two specification choices matter more than any single product label. First, keep the chemical out of the room. Filtration helps; source control helps more. Second, off-gassing is highest when the building is newest — often the same weeks schools, offices, and apartments first fill with people. Low-emitting materials, documented emissions data, and a flush-out with outdoor-air ventilation before occupancy are ordinary professional tools. An energy recovery ventilator makes that flush-out possible without wasting the heating and cooling budget.

Fine Particles and Cancer Risk

Mold and gases are not the only load. Fine particulate matter — PM₂.₅, particles 2.5 micrometers and smaller — can travel deep into the lung and into the blood.

A paper published in August 2026 in Nature Health estimated the global cancer burden associated with long-term air pollution. The authors examined 109 million cancer cases across 952 locations from 2000 to 2020. Each 10 µg/m³ increase in PM₂.₅ was associated with a 16 percent higher risk of all cancers. They attributed roughly 8.82 million incident cases in that period to PM₂.₅. Ozone and nitrogen dioxide added further burden. Women carried a higher attributable risk for most malignancies. The largest preventable fractions were in cancers of the respiratory, reproductive, and endocrine systems. About 17.9 percent of tracheal, bronchial, and lung cancers were estimated as preventable if PM₂.₅ exposure were removed.

That study measured outdoor air. It still belongs in a building conversation. Outdoor particles become indoor air unless the building filters them, and indoor sources — cooking, combustion, printers, disturbed dust — add more. A ventilation system with MERV 13 filtration on the outdoor-air stream reduces that dose. An unfiltered classroom on a busy road does not.

What Burns When the Room Catches Fire

Most contemporary furniture is polyurethane foam in a polyester cover, on engineered wood, on synthetic carpet. UL’s Fire Safety Research Institute ran side-by-side burns of rooms furnished with natural materials and rooms furnished with synthetics. Rooms with wood, cotton, and similar furnishings reached flashover in about 30 minutes. Matching rooms with synthetic furnishings reached flashover in roughly three and a half to five minutes. In one trial, the time was 3 minutes 20 seconds.

The synthetic rooms also produce heavier smoke and a different mix of toxic gases, including carbon monoxide and hydrogen cyanide. Some flame-retardant treatments slow flame spread but increase smoke toxicity. Polyurethane foam and polyester fabrics ignite and produce smoke much faster than the cotton and wood furnishings they replaced.

Architects and interior designers influence finish schedules, foam and fabric where the project allows it, compartmentation between units, and the paths that become smoke paths. Interior specifications have health consequences for off-gassing, toxicity, and flame spread during a fire.

Schools and Early Childhood

Classrooms are among the densest rooms we design regularly. Children breathe more air relative to body weight than adults. Their lungs and brains are still developing.

A review of more than 2,000 naturally ventilated primary classrooms found a median carbon dioxide level of about 1,487 ppm, with 81 percent of rooms above the commonly cited 1,000 ppm mark. Other classroom studies have recorded peaks near 5,000 ppm. When those rooms raised outdoor-air rates, children’s attention, vigilance, memory, and word recognition improved.

Elevated CO₂ is not oxygen starvation in the medical sense. Rooms rarely become hypoxic. It reliably signals stale, poorly diluted air carrying other things occupants produce, including respiratory viruses. Cognition drops. Infection risk rises. Asthma-related absences accumulate.

Younger children are more exposed still. Children under six have developing immune and respiratory systems, a higher surface-area-to-volume ratio, and faster respiration. Studies of nurseries routinely find CO₂ above 1,000 ppm. Code-minimum ventilation is a thin standard for a room of four-year-olds. It is better to design for continuous, filtered outdoor air, sized for actual occupancy.

Legionella in Larger Buildings

Legionnaires’ disease is a severe pneumonia caused by Legionella bacteria. People inhale contaminated water droplets; the illness does not spread person to person. The bacteria grow in warm water: cooling towers, large domestic hot-water systems, decorative fountains, and plumbing with poor circulation. Large buildings create those habitats.

This is a water-system problem that arrives as an air problem. Cooling-tower placement relative to outdoor-air intakes, dead legs in piping, lukewarm storage, and the quality of the maintenance plan are design and operations choices. Occupants do not experience mechanical disciplines separately. They experience the air.

What Balanced Ventilation Does in a Tight Building

A tight envelope without planned outdoor air holds whatever is generated inside. A tight envelope with balanced ventilation gives the building a controlled lung. 

An energy recovery ventilator pulls stale air out and brings in an equal volume of outdoor air. The two streams pass through a core that transfers heat and moisture without mixing. Passive House–certified equipment limits that cross-contamination to under 3 percent. Incoming air is filtered, typically to MERV 13 on the outdoor-air side.

Used together with an airtight, warm envelope, that system does several jobs at once. It continuously dilutes CO₂, VOCs, and occupant-generated pollutants. It helps manage indoor moisture so mold is harder to start. It filters outdoor particles before they become indoor particles. It makes winter and summer ventilation possible without the energy cost that leads owners to switch systems off. 

In multifamily buildings, offices, and schools, compartmentation is the other half of the strategy: treat each apartment or classroom as its own air volume so stack effect and shared corridors do not move air from unit to unit. Combined with unitized or carefully designed central ERVs, neighboring rooms stop serving as one another’s fresh-air source. That is a practical way to reduce airborne transfer between apartments and classrooms.

Measured Passive House dwellings, as a group, show lower pollutant and CO₂ levels than comparable naturally ventilated homes. Commissioning the ventilator and testing the envelope help explain why.

What can building owners do this year?

• Find the water first. Mold starts in the roof, windows, bathrooms, and HVAC coils.

• Measure the classroom or open office. A simple CO₂ monitor will show whether the ventilation you think you have is the ventilation you have. Sustained readings above 1,000 ppm mean the room is underserved (aim for 450–800 ppm).

• Specify the air, not only the appearance. Low-emitting finishes, furniture that is not a petroleum cushion where you can influence it, MERV 13 filtration, and an ERV that is commissioned and maintained will do more for occupant health than another interior gesture.

• On a busy corridor or a high-ozone afternoon, opening a window is not always the healthier move. Filtered, balanced ventilation is.

• The work is in the details: water management, planned outdoor air, and materials chosen for what they emit after they leave the showroom.

Next issue: water — the other half of mold, the whole of Legionella, and why a building’s plumbing is as much a health system as its façade. See HELD Architecture's site for other articles.

The top image is of the Pester PAC USA headquarters outside of Charlotte, NC. It was designed by HELD Architecture.