The Biology of Indoor Air: Why What You Breathe at Home Matters More Than You Think

The air inside your home is likely more polluted than the air outside. This counterintuitive fact holds true even in cities with significant outdoor air pollution. Buildings concentrate pollutants from construction materials, furnishings, cleaning products, and human activities while often providing inadequate ventilation to dilute them.

This matters because you breathe this air constantly. Every breath delivers whatever is suspended in your indoor environment directly into your lungs and, from there, into your bloodstream. The quality of that air affects your respiratory health, your cognitive function, your energy levels, and your long-term disease risk.

What's Actually in Your Indoor Air

Indoor air contains a complex mixture of particles, gases, and biological contaminants. Understanding what you're breathing is the first step toward improving it.

Volatile Organic Compounds (VOCs)

VOCs are gases emitted by a wide range of products, including paints, adhesives, furniture, flooring, and cleaning supplies. Common VOCs include formaldehyde, benzene, and toluene. These compounds can cause immediate symptoms like headaches and respiratory irritation, and some are associated with long-term health effects including cancer.

New buildings and recently renovated spaces typically have the highest VOC levels, as materials off-gas most intensively when new. But off-gassing can continue for months or years, and the cumulative effect of many low-emission sources can still create problematic indoor air.

Particulate Matter

Particulate matter includes dust, pollen, mold spores, pet dander, and fine particles from cooking and combustion. These particles vary in size, with smaller particles being more concerning because they can penetrate deeper into the lungs and even enter the bloodstream.

Indoor particulate levels are influenced by outdoor air quality, indoor activities, filtration systems, and cleaning practices. Cooking, particularly with gas stoves, can generate significant particulate matter. So can burning candles or incense.

Carbon Dioxide (CO2)

CO2 is a natural byproduct of human respiration. In well-ventilated spaces, CO2 levels remain low. In poorly ventilated spaces, particularly bedrooms with closed doors, CO2 can accumulate to levels that impair cognitive function and sleep quality. Elevated CO2 doesn't cause acute harm, but it's a reliable indicator of inadequate ventilation.

Biological Contaminants

Biological contaminants include mold, bacteria, viruses, and dust mites. These thrive in specific conditions: mold needs moisture, dust mites need humidity and organic material. Controlling these contaminants requires managing the indoor environment rather than just filtering the air.

The Cognitive Cost of Poor Air Quality

The effects of indoor air quality on cognitive function are well-documented but underappreciated. Studies have shown that elevated CO2 and VOC levels impair decision-making, reduce productivity, and increase errors. The effects are measurable at concentrations commonly found in offices and homes.

One landmark study found that cognitive function scores were 61% higher in green buildings with enhanced ventilation compared to conventional buildings. Another found that doubling ventilation rates in offices improved cognitive function by 8%. These are not small effects.

The implications for knowledge workers are significant. If you work from home, the air quality in your home office directly affects your cognitive performance. If you're trying to learn, create, or make important decisions, you're doing so with whatever cognitive capacity your indoor air allows.

Ventilation: The Foundation of Indoor Air Quality

The most effective way to improve indoor air quality is to increase ventilation, the exchange of indoor air with outdoor air. Ventilation dilutes indoor pollutants and prevents the accumulation of CO2 and other contaminants.

Modern buildings are often designed for energy efficiency, which means they're tightly sealed to prevent air leakage. This is good for heating and cooling costs but problematic for air quality unless mechanical ventilation is provided. Many homes rely on infiltration, air leaking through gaps in the building envelope, for their fresh air supply. This is neither reliable nor controllable.

The solution is intentional ventilation design. This might include exhaust fans in bathrooms and kitchens, fresh air intake in HVAC systems, or dedicated ventilation systems like energy recovery ventilators (ERVs) that bring in fresh air while recovering heat or cooling from exhaust air.

Opening windows is the simplest form of ventilation, but it's not always practical due to outdoor air quality, noise, security, or weather. A well-designed home should not depend on open windows for adequate air quality.

Source Control: Preventing Pollution at the Source

Ventilation dilutes pollutants, but source control prevents them from entering the air in the first place. This is often more effective and efficient than trying to remove pollutants after they're airborne.

Source control starts with material selection. Low-VOC paints, adhesives, and finishes are widely available and perform comparably to conventional products. Solid wood furniture off-gasses less than composite wood products. Natural fiber textiles are generally preferable to synthetic ones.

Source control also involves behavioral choices. Using exhaust ventilation while cooking, avoiding fragranced products, and maintaining appropriate humidity levels all reduce indoor air pollution.

Filtration: The Last Line of Defense

Air filtration removes particles and, with appropriate filters, some gases from indoor air. It's most effective for particulate matter and less effective for VOCs, which require activated carbon or other specialized media.

The effectiveness of filtration depends on the filter's efficiency, the amount of air passing through it, and how well the system is maintained. HEPA filters capture 99.97% of particles 0.3 microns and larger, making them effective for most indoor particulates. But a HEPA filter in a system that moves little air or is rarely replaced provides little benefit.

Portable air purifiers can improve air quality in individual rooms but are limited by their capacity. Whole-house filtration integrated into the HVAC system is more effective but requires appropriate system design.

Monitoring: What Gets Measured Gets Managed

Indoor air quality monitors have become affordable and accessible. These devices can track CO2, VOCs, particulate matter, temperature, and humidity, providing real-time feedback on indoor conditions.

Monitoring serves several purposes. It reveals problems you might not otherwise notice. It helps you understand how your activities and ventilation affect air quality. And it provides feedback on the effectiveness of interventions.

The goal is not to obsess over numbers but to develop an intuitive understanding of your indoor environment and ensure it supports rather than undermines your health.