The Lifecycle of Indoor Pollutants: VOCs, CO₂, PM, and Beyond

A scientific exploration of how indoor pollutants emerge, evolve, and persist within modern built environments

Abstract

Indoor pollutants are often discussed in isolation, yet each has a distinct lifecycle,  from source emission to human exposure and eventual removal or transformation. Understanding these lifecycles is essential to addressing not only pollutant concentration but also persistence, interaction, and health impact over time. This article analyzes the pathways of volatile organic compounds (VOCs), carbon dioxide (CO₂), particulate matter (PM), and bioaerosols within indoor spaces, drawing on peer-reviewed studies to map their emission patterns, behavior, accumulation dynamics, and mitigation challenges. The aim is to clarify how these pollutants function over time and why short-term measurements may fail to capture long-term risk.

1. Introduction

What happens to air pollutants after they enter or are generated within indoor spaces? This central question is often overlooked in indoor air quality (IAQ) assessments that emphasize instantaneous readings rather than pollutant trajectories over time. Yet the lifecycle of a pollutant,  from its source, through its transformation and transport, to its eventual decay or removal,  determines its real impact on occupant health. This concept is foundational in environmental science and toxicology, where persistence, bioaccumulation, and residence time often matter more than peak exposure. In enclosed indoor environments, the lack of natural dilution, ventilation constraints, and chemical surface interactions amplify the importance of lifecycle thinking. What follows is a pollutant-by-pollutant analysis of how indoor contaminants behave as systems rather than snapshots.

2. Volatile Organic Compounds (VOCs)

VOCs are carbon-based compounds that easily vaporize at room temperature. Common sources include synthetic materials, furniture adhesives, paints, varnishes, cleaning agents, office electronics, and even personal care products. The VOC lifecycle begins with off-gassing,  a diffusion-driven process where chemicals escape from solid or liquid matrices into ambient air. This emission is often non-linear, characterized by an initial high-rate "burst phase" followed by slower continuous release that can persist for months or even years. Formaldehyde, for example, may off-gas from particleboard for over five years, especially in humid conditions. Once airborne, VOCs may adsorb to indoor surfaces like fabrics, carpets, or walls, creating secondary emission reservoirs that release compounds when temperatures or humidity increase. Moreover, VOCs can undergo chemical reactions with ozone or hydroxyl radicals, forming secondary pollutants such as formaldehyde or ultrafine particles. Studies by Nazaroff and Weschler (2004) emphasize the formation of these secondary organic aerosols (SOAs) as a major indoor chemistry concern. These transformations mean that even if a primary VOC is removed, its reactive byproducts may persist or pose new hazards.

3. Carbon Dioxide (CO₂)

Unlike VOCs, CO₂ is not a toxin at normal concentrations but becomes problematic due to its physiological effects at elevated levels. The indoor lifecycle of CO₂ is tightly coupled to human occupancy and ventilation rate. CO₂ is continuously generated by respiration, with each adult emitting 0.3 to 0.4 liters per minute. In inadequately ventilated spaces, CO₂ accumulates quickly, often exceeding 1000 parts per million (ppm), which is associated with reduced cognitive function, drowsiness, and impaired decision-making. Allen et al. (2016) found that even at 950 ppm,  well below OSHA safety limits,  participants performed significantly worse on complex cognitive tasks. Importantly, CO₂ does not settle or degrade but is diluted through air exchange. Thus, without real-time ventilation or active biosequestration mechanisms, CO₂ levels remain elevated for hours or even days, especially in high-occupancy buildings like classrooms and meeting rooms. The CO₂ lifecycle is therefore cyclical, reflecting human behavior patterns and the dynamic between emission and ventilation.

4. Particulate Matter (PM)

Particulate matter includes solid and liquid particles suspended in air, ranging from PM10 (dust and pollen) to PM2.5 and PM1 (combustion particles, ultrafine particles). Sources include cooking, smoking, candles, incense, printers, and infiltration from outdoor traffic or construction. Once released, PM does not behave like a gas. Its fate is governed by gravitational settling, surface deposition, re-suspension by movement or cleaning, and in some cases, agglomeration into larger particles. PM2.5 can remain airborne for hours and is capable of penetrating deep into alveolar tissue, bypassing mucosal filtration. PM1 and ultrafine particles, due to their small size, exhibit Brownian motion and can remain suspended indefinitely in still air. Studies by Morawska et al. (2013) highlight the fact that PM indoors is highly dynamic, with concentrations fluctuating rapidly in response to occupant movement and ventilation shifts. Once deposited on surfaces, particles can be re-aerosolized during vacuuming, walking, or furniture use, creating secondary exposure events. Therefore, PM has both an airborne and surface lifecycle, necessitating cleaning strategies that address both air and contact surfaces.

5. Bioaerosols and Microbial Volatiles

Biological particles,  fungal spores, bacteria, viruses, skin flakes, pollen, and microbial VOCs,  form a significant but often underappreciated class of indoor pollutants. Their lifecycle includes emission from humans, pets, HVAC systems, or damp materials, airborne transport, and interaction with building microclimates. Unlike chemical pollutants, bioaerosols can reproduce under favorable conditions. High humidity, for instance, promotes mold growth on surfaces that then release spores or microbial VOCs. These byproducts, including compounds like 1-octen-3-ol and geosmin, are linked to irritation, fatigue, and even cognitive impairment. A study published in Indoor Air (2009) found that buildings with visible mold growth had significantly higher rates of reported fatigue, memory problems, and respiratory distress. Because bioaerosols are living or semi-living systems, they have nonlinear lifecycles involving latency, amplification, and reactivation. This creates a risk not just of steady-state exposure but of sudden bloom events that standard air sensors may not detect in time.

6. The Challenge of Measurement and Misinterpretation

One of the greatest challenges in addressing the lifecycle of indoor pollutants is the temporal bias of IAQ measurement. Most IAQ audits rely on short-duration spot checks or average daily values. These snapshots fail to capture peak accumulation, decay phases, secondary pollutant formation, or re-emission events. For example, a room cleaned with a disinfectant may show low VOC levels one hour later, but as the surface dries and warms, VOC release may spike hours afterward. Similarly, PM readings taken after hours of stillness may appear low, even though occupant movement during the day would dramatically increase exposure. Without high-resolution, time-series data and lifecycle modeling, decision-makers risk both underestimating and misdiagnosing indoor air quality problems.

7. Conclusion- Lifecycle of Indoor Pollutants

Indoor pollutants are not static entities; they are dynamic agents with lifecycles that span emission, transformation, interaction, and re-emission. Understanding their behavior requires more than knowing their concentration at a moment in time. It requires studying how they move, react, persist, and evolve within the complex ecosystem of indoor environments. From VOCs that off-gas for years, to CO₂ that builds up in tandem with human presence, to PM and bioaerosols that circulate invisibly but biologically, the science of indoor air demands a lifecycle approach. Only by mapping these lifecycles can we design interventions,  biological, mechanical, or architectural,  that are responsive, predictive, and resilient. Lifecycle thinking is not only the next frontier of IAQ science; it is the necessary foundation for designing buildings that are truly breathable, livable, and safe.

To learn how nature-based systems are being applied to respond to pollutant lifecycles in real time, visit: www.justbreathe.in