Inside the Air We Breathe: Two-Year Home Monitoring Reveals Hidden Pollution Exposures

Most of what we know about the health dangers of air pollution comes from monitors bolted to rooftops and roadside stations, devices that measure the air outside our homes. Yet people in modern societies spend between 80 and 90 percent of their lives indoors, much of that time inside their own residences. For vulnerable groups such as the elderly and those with chronic lung disease, the share is even higher. A new study published in the journal Environmental Health argues that this mismatch between where pollution is measured and where people actually breathe it has quietly distorted decades of epidemiological research, and it offers one of the most detailed blueprints yet for fixing the problem.

The research, led by Sun-Young Kim of the National Cancer Center Graduate School of Cancer Science and Policy in South Korea, describes a two-year indoor air monitoring campaign covering 130 older adults with idiopathic pulmonary fibrosis, a progressive and often fatal scarring of the lungs, living in the Seoul Metropolitan Area. The region, home to roughly 26 million people, is the fourth largest metropolitan area in the world, and its ambient fine particulate matter concentrations average around 21 micrograms per cubic meter, well above the 12 micrograms per cubic meter guideline set by the World Health Organization. Between July 2021 and August 2023, the team repeatedly placed lightweight sensors inside participants’ homes to capture the air these patients actually breathed, season after season.

The technical heart of the campaign was the RTI MicroPEM, a portable particulate matter sensor weighing about 230 grams and barely louder than office background noise. Unlike the cheap optical sensors common in consumer air quality gadgets, the MicroPEM does something more powerful: while it records real-time particle concentrations every 30 seconds, it simultaneously collects particles on a PTFE filter that can be weighed in the laboratory for gravimetric accuracy and analyzed for chemical composition. The researchers used the gravimetric measurements to correct the optical readings, then averaged the adjusted data to one-minute resolution. Filters were analyzed for black carbon, brown carbon, and environmental tobacco smoke using six-wavelength integrating sphere photometry, and for 33 inorganic elements by X-ray fluorescence, yielding a chemical fingerprint of each home’s air.

Four design principles shaped the study. First, the team wanted measurements that genuinely represented long-term exposure rather than a snapshot, so each home was sampled for five consecutive days, spanning weekdays and weekends, in each of the four seasons. Second, they sought to identify the sources of indoor pollution, combining a household questionnaire of more than 100 questions with the chemical composition data. Third, they needed to quantify how much of the indoor particles had drifted in from outdoors, a question of infiltration that matters enormously in dense cities. Fourth, they wanted to minimize the burden on participants, developing a non-contact protocol in which sensors were shipped to and from homes by postal mail, an approach born of necessity during the COVID-19 pandemic and refined with detailed written instructions and follow-up phone calls for older adults.

The early findings are striking. Across 579 valid filter samples and more than 62,000 hourly measurements, the mean indoor hourly concentration of fine particulate matter was 14.90 micrograms per cubic meter, with a standard deviation of 36.12 reflecting sharp spikes from indoor activities. Winter concentrations, averaging 20.17 micrograms per cubic meter, were nearly double the summer average of 11.19, a seasonal contrast the researchers attribute to heating in cold months and rain-scrubbed air in humid summers. Concentrations peaked during morning and evening hours between 7 and 10 a.m. and 6 and 8 p.m., precisely when cooking and cleaning activities occur. Notably, indoor levels were consistently lower than matched outdoor measurements, which averaged 20.44 micrograms per cubic meter, yet the hourly patterns ran in opposite directions: outdoors peaked at night while indoors peaked during the day.

The chemical analysis added a second layer of insight. Carbonaceous components, including black carbon at 0.57 micrograms per cubic meter, brown carbon at 1.27, and environmental tobacco smoke markers at 2.01, alongside 33 inorganic elements led by sulfur at 0.89, together made up about half of the total particulate mass, with the fraction varying from 34 percent in summer to 60 percent in winter. By comparing indoor and outdoor concentrations of elements with few indoor sources, the team estimated infiltration factors. Median indoor-to-outdoor ratios for sulfur, nickel, and iron ranged from 51 to 70 percent, meaning that between roughly half and two-thirds of outdoor fine particles penetrate into homes, a figure that varies with building characteristics, ventilation behavior, and season.

Household characteristics told their own story. About 85 percent of participants spent more than 15 hours at home, nearly all opened windows for ventilation, three-quarters used air cleaners, and almost all cooked with natural gas. Average particulate concentrations were higher in homes with cooking activities, 15.14 versus 10.96 micrograms per cubic meter, in homes with humidifiers, 19.19 versus 13.16, in homes with pets, 24.09 versus 13.35, and in homes with smokers, 16.76 versus 11.79. Interestingly, air cleaners showed little overall effect, though households with multiple units trended toward lower concentrations, and homes combining mechanical ventilation with open windows fared better than those relying on windows alone.

Why does all this matter for health? Idiopathic pulmonary fibrosis has few identified risk factors beyond smoking, and recent studies have implicated air pollution in disease progression and mortality, possibly through mechanisms involving oxidative stress, inflammation, and telomere shortening. Previous indoor studies, including work on chronic obstructive pulmonary disease patients in Massachusetts, found that weekly average indoor fine particle and black carbon concentrations predicted lung function decline more strongly than outdoor levels did. But most prior indoor monitoring covered only hours, days, or a single season, and the well-known SPIROMICS AIR study, though it extended monitoring to two weeks, still sampled only two or three seasons. By capturing all four seasons across more than two years, the Korean campaign achieves something rare: a genuine estimate of each individual’s annual average indoor exposure.

The study also demonstrates that the logistical barriers that have long stymied residential monitoring can be overcome. Of the 130 participants, 95, or 73 percent, completed all four seasonal visits, a remarkable retention rate for a population of mostly retired adults in their late sixties. The postal-mail protocol eliminated most in-person contact, and the sensors’ five-to-six-day battery life avoided midweek interventions. The researchers suggest that even two seasons, one high-concentration and one low-concentration, may approximate annual averages with acceptable error, opening the door to cheaper, streamlined designs. Their next steps include building prediction models to estimate indoor exposures for people without direct measurements, applying source apportionment to the chemical data, and extending the framework to other vulnerable groups such as asthmatic children and cancer patients. As the evidence linking indoor air to chronic disease accumulates, this study provides both a warning, that rooftop monitors may misrepresent the air we actually breathe, and a practical roadmap for measuring the pollution that matters most.

Subject of Research: Long-term residential indoor exposure to fine particulate matter air pollution in susceptible populations for epidemiological health research

Article Title: Residential indoor air monitoring to assess long-term exposure to air pollution in susceptible populations for epidemiology: overview and early findings

Article References: Kim, S.-Y., Kim, K., Ji, S., Shin, M., Kim, H., Park, Y. S., Han, S., Bae, S., Kim, Y., Park, E., Hwang, J., Cho, S.-H., Yoon, H.-Y., & Song, J. W. (2026). Residential indoor air monitoring to assess long-term exposure to air pollution in susceptible populations for epidemiology: overview and early findings. Environmental Health, 25(1), Article 77. https://doi.org/10.1186/s12940-026-01334-3

Image Credits: AI Generated

DOI: 10.1186/s12940-026-01334-3

Keywords: indoor air pollution, PM2.5, exposure assessment, epidemiology, idiopathic pulmonary fibrosis, air quality monitoring, infiltration, MicroPEM, Seoul Metropolitan Area, chemical components, susceptible populations, Environmental Health