The Chemistry of VOCs, Ozone, Particulates & Household Materials

Indoor air pollution is often discussed in terms of individual contaminants—volatile organic compounds (VOCs), particulate matter (PM), ozone (O₃), mold spores, allergens, carbon monoxide, or nitrogen dioxide. But real indoor environments are far more complex than isolated pollutants. Homes contain thousands of chemical compounds interacting with one another continuously. These interactions create new secondary pollutants that may be more irritating or harmful than the original sources. Understanding how pollutants react, transform, or combine is central to controlling indoor air quality effectively.

The human nose provides clues to these interactions. A room might smell clean after using a scented cleaner but actually contain higher levels of VOCs and secondary byproducts. Sunlight entering a room can trigger reactions between surface residues and ozone, creating odors or irritants. Cooking emissions combine with household dust to form sticky particulate films. Even fragrances from candles or sprays can react with pollutants already in the home, forming formaldehyde or secondary organic aerosols (SOAs).

Indoor air chemistry is dynamic because pollutants do not remain in their original form. VOCs from cleaning products, furniture, and paints evaporate into the air; ozone enters when windows are open or is generated by certain electronic devices; particulates from cooking or dusting remain airborne or settle on surfaces; biological fragments enter from humans, pets, and microbes. When these pollutants coexist, they undergo chemical reactions, adsorption onto surfaces, transformation through oxidation, or resuspension from dust. These processes reshape the entire indoor environment and influence how clean, fresh, or irritating the air feels.

VOCs represent one of the largest contributors to indoor chemical reactions. They evaporate from a vast range of sources—furniture coatings, adhesives, paints, cleaning sprays, cosmetics, flooring materials, plastics, air fresheners, and household products. Some VOCs dissipate quickly; others linger in carpets, drywall, fabric, or dust for months. When VOC molecules encounter ozone, they oxidize, producing aldehydes, organic acids, peroxides, and fine particles. In fact, ozone–VOC reactions are responsible for most secondary indoor pollutants found in modern homes.

Ozone behaves differently indoors than outdoors. Outdoor ozone enters through open windows or gaps but is quickly depleted as it reacts with surfaces and indoor chemicals. Yet even low levels of ozone can react strongly with VOCs emitted from everyday activities. Terpenes—natural fragrances found in citrus cleaners, pine oils, scented candles, and air fresheners—are particularly reactive with ozone. These reactions produce secondary organic aerosols (SOAs), which are microscopic particles that remain airborne and can irritate the respiratory system.

Particulate matter (PM) also plays a crucial role in indoor air chemistry. Particles act as carriers for chemicals, allergens, microbes, and VOC residues. They provide surfaces where chemical reactions occur, acting as platforms for ozone reactions or VOC adsorption. Cooking emissions generate ultrafine particles that remain airborne for hours and combine with pre-existing particles from dust or smoke. These mixed particles form sticky films on walls, ceilings, cabinets, and electronics—a residue commonly known as “thirdhand smoke” in homes where tobacco is used.

To understand these complex interactions more clearly, the following list outlines the primary pollutant categories and how they chemically interact inside homes.

Major Indoor Pollutants & How They Interact

• VOCs (from cleaners, paints, furnishings): react with ozone to form aldehydes, SOAs, and irritants
• Ozone (outdoor infiltration or electronic devices): oxidizes VOCs, degrades rubber and fabrics
• Particulates (PM2.5, ultrafine particles): combine with VOCs, oils, and dust to create sticky films
• Microbial VOCs (mold, bacteria): blend with household VOCs, intensifying odors
• Nitrogen dioxide (from gas stoves): promotes oxidative chemistry with VOCs
• Surface residues (on walls, carpets, countertops): off-gas or re-emit when heated or touched
• Humidity: accelerates chemical reactions and increases reactivity of certain compounds

Humidity is a powerful accelerator of indoor chemical processes. High humidity facilitates hydrolysis reactions, increases the solubility of gases in water films on surfaces, and promotes mold growth, which introduces microbial VOCs (mVOCs) into the air. Humidity also increases the mobility of semi-volatile organic compounds (SVOCs), allowing them to migrate from surfaces into the air. When humidity rises, carpets, fabrics, and dust absorb more water vapor, making them active platforms where chemicals accumulate or react.

Another significant factor influencing indoor chemistry is sunlight. Sunlight entering a home through windows can trigger photochemical reactions on surfaces. VOCs deposited on walls or furniture may undergo oxidation when exposed to UV light, generating new odor compounds or aerosols. Sunlight can also break down ozone, creating reactive radicals that initiate additional chemical pathways. This is why rooms exposed to strong sunlight often develop distinct “warm” odors that differ from shaded rooms.

Pollutants also interact with surfaces. Indoor surfaces are not inert; they act as chemical sinks that absorb, transform, or re-emit pollutants. Carpets trap VOCs, which then slowly re-evaporate. Painted walls absorb ozone and VOCs and release secondary compounds hours later. Curtains and fabrics collect particulate matter, oils, and chemical residues. When heated by sunlight or airflow, these materials re-emit pollutants into the room. Wood, drywall, and insulation absorb moisture and allow microbial VOCs to form when humidity is high.

These processes fundamentally shape indoor air quality. Homes with poor ventilation allow pollutants to accumulate, giving chemical reactions more time and opportunity to occur. Homes with high ventilation rates dilute pollutants and slow down reaction pathways. HVAC systems influence the movement of particles and VOCs by circulating air, filtering particulates, and controlling temperature and humidity. When HVAC filters are dirty, particles accumulate and increase chemical reaction surfaces.

Understanding how pollutants mix and react is essential for controlling indoor air quality. The next list outlines the most important strategies to reduce pollutant interactions and limit secondary chemical formation.

How to Minimize Harmful Indoor Pollutant Interactions

• Increase ventilation to dilute indoor pollutants and reduce ozone–VOC reactions
• Use low-VOC and fragrance-free products to minimize terpene–ozone chemistry
• Run kitchen exhaust fans when cooking to remove combustion particles
• Avoid ozone-generating devices such as certain “air purifiers”
• Keep humidity between 30–50% to limit chemical reactivity and microbial growth
• Clean dust regularly with microfiber cloths to remove reaction surfaces
• Use HEPA filtration to reduce particulate matter that participates in chemical reactions
• Vent bathrooms and laundry rooms to reduce moisture that encourages chemical reactions
• Store chemicals tightly sealed to reduce continuous VOC emissions
• Avoid burning candles or incense, which introduce reactive VOCs and particulates
• Open windows during and after cleaning to reduce VOC buildup

Cooking deserves special attention because it produces a unique blend of particulates and VOCs that undergo chemical reactions in the air. Frying releases oils that oxidize quickly, forming secondary organic aerosols. Gas cooking emits nitrogen dioxide, which accelerates oxidation processes. Even electric stoves produce particulate emissions from food residues. Without strong ventilation, these pollutants persist and interact with dust, humidity, and household VOCs.

Cleaning products are another major contributor to indoor chemistry. Many “fresh” or citrus-scented cleaners contain terpenes such as limonene or pinene. These compounds smell pleasant but react aggressively with indoor ozone. When this reaction occurs, it can create formaldehyde—a known irritant and carcinogen—and a range of fine particles. These reactions explain why homes sometimes smell “sharp” or “chemical” after cleaning, even if the home looks spotless.

Scented candles, plug-in fragrances, incense, and essential oils all introduce terpenes and SVOCs into the air. Although marketed as “natural,” essential oils are chemically active and contribute to ozone–VOC reactions that form secondary pollutants. Burning candles adds soot and ultrafine particles, which further increase chemical reaction surfaces.

Ozone-generating devices marketed as “air purifiers” create more harm than good. Ozone reacts with nearly every organic material in a home—rubber, paint, fabrics, wood finishes—forming harmful byproducts. Even low ozone levels degrade materials and create irritants.

The interactions between pollutants become more dramatic in tightly sealed modern homes. While energy-efficient construction reduces heat loss, it also reduces natural air exchange. Pollutants persist longer, accumulate in higher concentrations, and undergo more chemical transformations. Homes with insufficient ventilation or stagnant airflow develop a cocktail of secondary pollutants that may cause irritation, headaches, fatigue, or respiratory discomfort—even when the source activities seem benign.

Ultimately, improving indoor air quality is not just about reducing a single pollutant—it’s about managing the entire system of chemical interactions. Ventilation dilutes pollutants and slows secondary reactions. Clean surfaces minimize reaction platforms. Humidity control reduces chemical mobility. Avoiding fragranced or terpene-heavy products reduces ozone interactions. Using high-quality HVAC filters reduces particulate surfaces where chemical transformations occur. When all of these strategies are combined, indoor chemistry becomes more stable and less reactive.

Healthy indoor air depends on understanding that pollutants don’t exist in isolation. Homes are dynamic chemical environments where thousands of compounds interact every moment. By controlling emissions, improving ventilation, reducing humidity, and choosing materials wisely, homeowners can dramatically reduce harmful chemical reactions and improve the freshness, safety, and stability of indoor air.

The combination of good ventilation, low-VOC products, proper humidity control, and regular dust removal forms the foundation of an indoor environment where pollutants are less likely to interact and transform into more harmful compounds.

Scientific Sources

1. Environmental Protection Agency (EPA) – “Indoor Air Chemistry & VOC Interactions”
   https://www.epa.gov/indoor-air-quality-iaq
2. ASHRAE – Indoor Aerosols, VOCs & Oxidative Chemistry Guidelines
   https://www.ashrae.org
3. National Institutes of Health (NIH) – Indoor Secondary Organic Aerosol Formation Research
   https://pubmed.ncbi.nlm.nih.gov