Biological contaminants — including bacteria, viruses, allergens, and mold — are key determinants of indoor air quality and respiratory health. Their presence and impact depend on moisture, ventilation, and maintenance, making environmental control essential to reducing exposure and preventing disease.

Bacteria and viruses are widespread biological agents that originate primarily from human activity. They enter the air through coughing, sneezing, breathing, and speaking, remaining suspended for minutes to hours depending on ventilation and environmental conditions. These microorganisms contribute to respiratory and allergic disease, with transmission shaped by occupancy, crowding, and maintenance of ventilation and hygiene systems.

Allergens are proteins and glycoproteins released by mites, pets, pests, fungi, and pollen that become airborne and settle into dust and fabrics. These substances persist in indoor environments and are easily resuspended through everyday activity, leading to continuous exposure in homes and schools. They contribute to asthma, rhinitis, and other allergic conditions, with levels influenced by humidity, ventilation, and cleaning practices.

Mold consists of fungi that release microscopic spores and colonize damp materials such as drywall, wood, and insulation. Growth occurs only when moisture is present, allowing colonies to spread through ventilation systems and surfaces. Exposure is associated with asthma, wheeze, and respiratory irritation, particularly in children and occupants of damp or poorly ventilated buildings. Because mold thrives in persistent moisture, prevention depends on humidity control, prompt leak repair, and regular maintenance.

The following subsections examine each of these major biological contaminants in detail:

3.1.1 Bacteria and Viruses

Bacteria and viruses are persistent components of indoor environments, linking indoor air quality directly to infectious and allergic disease. Managing ventilation, hygiene, and system maintenance is critical to reducing their spread and protecting health.

Bacteria and viruses are biological contaminants that play a major role in IAQ. Though distinct in biology — bacteria are free-living, single-celled organisms, while viruses are non-living particles that require a host cell to replicate — both can spread through the air and contribute to infectious disease. Indoors, activities such as coughing, sneezing, breathing, and speaking generate aerosols that remain suspended for minutes to hours, while their overall abundance is shaped by occupancy, ventilation, and environmental factors such as temperature and humidity.

Airborne bacteria and viruses have been linked to a wide range of health effects, from mild irritation to serious infection. Studies in hospitals and classrooms have identified common respiratory pathogens such as Staphylococcus aureus, Escherichia coli, influenza viruses, rhinoviruses, and coronaviruses, all of which can spread efficiently through indoor air when ventilation is limited. In healthcare settings, these microorganisms pose particular risks for immunocompromised patients, while in schools and libraries they contribute to respiratory and allergic symptoms among occupants. Broader reviews confirm that exposure to microbial contaminants and their byproducts can cause asthma, bronchitis, hypersensitivity pneumonitis, and other inflammatory responses, with temperature and humidity influencing their survival and infectivity.

Indoor transmission dynamics are shaped by building conditions. Ventilation rates consistently emerge as the most important determinant of microbial load, with poorly ventilated spaces — such as naturally ventilated offices, classrooms, and dwellings — showing higher bacterial and viral concentrations than those served by mechanical systems. Human occupancy and activity contribute the majority of airborne microorganisms through shedding and resuspension from skin and surfaces, while HVAC systems can accumulate and redistribute bacterial and viral material if not properly maintained. Metagenomic analyses have revealed that both bacterial and viral communities in indoor air reflect the people who occupy the space, varying across rooms and seasons.

Research shows that bacteria and viruses are widespread in indoor air, with their abundance and diversity reflecting how buildings are occupied and maintained. Because ventilation, hygiene, and system upkeep influence microbial accumulation and spread, effective management is essential to reducing transmission and protecting health.

3.1.2 Allergens

Allergens are common indoor contaminants that degrade air quality and contribute to respiratory irritation and disease. Managing humidity, ventilation, and cleaning practices is essential to reduce accumulation and protect health.

Allergens are biological contaminants that play a central role in IAQ and allergic disease. They are diverse proteins and glycoproteins shed by dust mites, pets, rodents, cockroaches, fungi, and pollen, all of which can become airborne and readily inhaled. Indoors, routine human activity can resuspend settled dust, while fabrics, carpets, and HVAC filters act as reservoirs and distribution pathways. Because these allergens persist in air and settled material, exposure occurs continuously in homes, schools, and child-care centers; even in spaces without direct sources, they can spread through air movement or be transported on clothing and hair.

Airborne allergens are strongly associated with respiratory and immune disorders, ranging from mild irritation to chronic allergic disease. Major indoor sources provoke rhinitis, asthma, and conjunctivitis in sensitized individuals. In children, early and repeated exposure increases the likelihood of sensitization and persistent wheezing, while in allergic adults it contributes to symptom severity and asthma exacerbations. Population studies show that elevated concentrations of mite, cat, and dog allergens correspond with higher rates of respiratory and skin symptoms, and that co-exposure to PM_2.5 or dampness amplifies these effects. Broader reviews confirm that allergen exposure underlies much of the global burden of asthma and allergic disease, with humidity, season, and building conditions influencing both allergen survival and potency.

Indoor exposure is nearly universal, with most homes and schools containing measurable levels of dust-mite, pet, and insect allergens. National and regional surveys consistently detect multiple allergen types in more than 80% of dwellings, and concentrations often exceed sensitization thresholds even in buildings without pets or visible contamination. Classrooms and child-care centers frequently show higher animal and mite allergen levels than homes, reflecting greater occupancy, carpeting, and cleaning frequency. Pollen allergens also infiltrate readily from outdoors and remain airborne as fine fragments long after the primary season. Indoor concentrations vary widely by region and building type, reflecting differences in occupant activity and source distribution.

Environmental and building conditions shape how allergens accumulate and persist indoors. Humidity, ventilation, surface materials, and cleaning frequency influence levels by affecting both particle generation and removal. Carpets, upholstery, and curtains act as reservoirs where allergens settle and can be resuspended with activity; classrooms with more textiles or open shelving show higher settled dust and pet allergens, while frequent cleaning reduces cat and dog allergens. In schools and child-care centers, lower ventilation rates and carpeting are associated with higher mite and animal allergens, whereas sound HVAC operation and regular cleaning are linked to lower levels. Ventilation rate governs dilution of airborne particles, and filters and air conditioning units can accumulate allergen reservoirs without routine maintenance. Elevated humidity supports dust-mite survival, and seasonal changes and occupancy patterns modulate concentrations across rooms and buildings.

Studies indicate that allergens are present in nearly all indoor environments, with their persistence determined by sources, materials, and occupant activity. Humidity control, ventilation, and cleaning reduce accumulation and limit the health impacts of long-term exposure.

3.1.3 Mold

Mold is a pervasive group of fungi closely tied to moisture and ventilation conditions. Maintaining building integrity is key to preventing growth and safeguarding respiratory health.

Mold is a diverse group of fungi that release microscopic spores into the air and readily colonize damp indoor materials such as drywall, wood, and insulation. While spores are always present, growth occurs only when moisture and organic matter are available. Surveys across North America and Europe show that visible mold and dampness are widespread in homes, schools, and workplaces, affecting roughly one in five dwellings. More than a thousand fungal species have been identified indoors, with Aspergillus, Penicillium, Cladosporium, and Stachybotrys among the most common. These organisms spread through ventilation systems and settle on surfaces, forming reservoirs that persist until moisture is controlled.

Indoor mold is consistently associated with adverse respiratory and allergic outcomes. Meta-analyses and large cohort studies show 30–80% higher odds in damp or mold-affected homes, with the strongest effects observed in children. Mold exposure has also been linked to respiratory tract infections and new-onset wheezing in a dose-dependent manner. Evidence indicates that fungi such as Aspergillus and Penicillium contribute to these outcomes through allergenic and inflammatory mechanisms, and that chronic exposure to spores and hyphal fragments may sustain low-grade airway inflammation. Overall, the evidence supports controlling visible dampness and mold as a primary strategy to reduce respiratory and allergic disease.

Quantifying mold exposure is challenging because airborne concentrations fluctuate sharply by season and even by day. Studies show spore counts can vary more than twentyfold across seasons, and total spore numbers — especially of Aspergillus and Penicillium — most effectively distinguish contaminated spaces from clean ones. Visible mold growth is a consistent indicator of elevated fungal loads, whereas MVOCs and self-report surveys provide limited accuracy and are often affected by bias. Because of this variability, single measurements rarely represent long-term exposure. Standardized sampling protocols and objective environmental measurements are preferred for research and remediation planning.

Indoor mold levels are shaped by building characteristics, operation, and maintenance. Higher concentrations are typically found in older dwellings, in homes without air conditioning, and in buildings with persistent humidity or limited ventilation. Socioeconomic factors also correlate with moldiness through differences in housing condition, age, and access to mechanical cooling. Pets, carpeting, and infrequent cleaning increase contamination, while mechanical ventilation with HEPA filtration substantially lowers indoor fungal concentrations in clinical settings. Construction practices and HVAC maintenance further influence long-term IAQ, as mold growth can begin during building assembly or within air handling units where moisture persists. Across climates, prevention relies on controlling humidity, ensuring adequate ventilation, and repairing leaks or drying damp materials within 24–48 hours to prevent colonization.

Evidence links indoor mold directly to building moisture, ventilation, and upkeep, making prevention a core aspect of healthy indoor environments. Timely repair of leaks, effective humidity control, and regular maintenance are essential to limiting fungal growth and reducing respiratory risks.