The association between air pollution and adverse health outcomes has been long established, but the causal mechanism(s) behind this association are still uncertain (1–4). Recent studies propose that an oxidant component of particle air pollution, environmentally persistent free radicals (EPFRs), may serve as an inducer of oxidative stress (OS), potentially bridging the gap between air pollution and its detrimental health impacts (5–14). EPFRs are formed during combustion processes and are typically associated with fine particulate matter (PM_2.5; particles with a mass median aerodynamic diameter of ≤2.5 μm). These radicals are commonly produced from sources such as traffic-related air pollution (TRAP), residential combustion activities, cooking, industrial burning, cigarette smoking/vaping, and wildfires (6, 10–12, 14–17). EPFRs are particularly concerning as they are free radicals that can persist in the environment and biological systems for prolonged periods (8, 10–12, 14). Typically, free radicals have short lifetimes—only a few picoseconds—due to the extreme instability of unpaired electron/s in their outermost orbit (8, 11); however, EPFRs are unusually stable and long-lasting. This stability arises from the continuous cycling of their valence electrons between paired and unpaired states, facilitated by electron transfer from organic particles to the surfaces of redox-active transition metals present in particulate matter (PM) (6, 7). As a result, these stabilized EFPRs can persist for extended periods, lasting up to 5 years in certain environments, and are notably resistant to decomposition (8, 12, 14).

Little is known about the presence of EPFRs in the household environment. Studies that have explored household environments mostly focused on airborne EPFRs produced from solid fuel combustion used for cooking and heating, demonstrating household practices can be important sources for residential EPFRs (13, 14, 18–20). In addition, motor vehicle emissions, a major source of ambient EPFRs, have been extensively reviewed (16, 21, 22). Household practices such as air-conditioning and window opening can influence TRAP penetration into indoor environments (23). Consequently, it is believed that EPFRs generated from TRAP can accumulate in homes as house dust through the infiltration of ambient air. Emerging evidence shows the potential for adverse health effects of household EPFRs (10), particularly for children, due to their increased sensitivity to exposures and continuing respiratory development (24). Sly et al. reported that 89 out of 90 house dust samples in Brisbane, Australia, had detectable EPFRs; high concentrations (≥ 6 × 10¹⁷ spins/g) were associated with an increased risk of wheezing in young children compared to low EPFR concentrations (< 4 × 10¹⁷ spins/g) (10). The health concerns from EPFR exposure highlight the importance of understanding the factors that influence the presence of EPFRs in indoor environments, particularly in residential homes. We hypothesize that household characteristics and behaviors influence EPFR concentration in house dust. This study aimed to determine household characteristics associated with EPFRs in Australian homes.

The EPFRs, air pollution, and household characteristics are shown in Tables 1, 2. The sample size for complete cases in the ELLF cohort was 46 homes, and in the BIS cohort, it was 136 homes. The median (interquartile range, IQR) EPFRs detected in the ELLF and BIS households were 1.65 × 10¹⁷ spins/g (IQR = 3.60 × 10¹⁷ spins/g) and 5.78 × 10¹⁶ spins/g (IQR = 1.21 × 10¹⁷ spins/g), respectively (Table 1). Based on the violin plots in Figure 1, the distribution of EPFR concentration in the ELLF cohort is much more spread out than in the BIS cohort, indicating greater variability in the Brisbane region. The EPFR concentration measured in both cohorts was below the low concentration group (< 4 × 10¹⁷ spins/g), as reported by Sly et al. (10), which found a protective association against wheezing in young children when compared to the high concentration group (≥ 6 × 10¹⁷ spins/g). Seasonal differences regarding EPFR concentration were inconsistent between the two geographical cohorts. In the ELLF cohort, lower median EPFRs were seen in warmer months of summer (7.06 × 10¹⁴ spins/g, IQR = 1.83 × 10¹⁷ spins/g) in comparison to colder months of winter (3.10 × 10¹⁷ spins/g, IQR = 3.13 × 10¹⁷ spins/g) (Figure 1). In the BIS cohort, there were slightly greater median EPFRs in summer (6.31 × 10¹⁶ spins/g, IQR = 4.53 × 10¹⁶ spins/g) when compared to winter (5.74 × 10¹⁶, IQR = 1.20 × 10¹⁷ spins/g) (Figure 1). However, the violin plots demonstrated longer upper tails in winter for both cohorts, indicating that the highest concentrations of EPFR tend to occur during colder weather.

Annual air pollution exposure was similar for both cohorts. The average annual ambient PM_2.5 concentration was 6.13 μg/m³ (SD = 0.32 μg/m³) in the ELLF cohort and 7.25 μg/m³ (SD = 0.69 μg/m³) in the BIS cohort. For annual average NO₂, concentrations were 12.25 μg/m³ (SD = 3.97 μg/m³) in ELLF and 5.36 μg/m³ (SD = 2.08 μg/m³) in the BIS cohort. In the ELLF cohort, the monitored 24-h average PM_2.5 concentrations were 13.55 μg/m³ indoors and 11.63 μg/m³ outdoors.

EPFRs are a relatively recent discovery, and research on residential exposure remains limited. To the best of our knowledge, the present study is the first to explore household characteristics associated with EPFR concentrations in house dust. In the ELLF study, we identified seven household characteristics or practices associated with EPFR concentrations, and in the BIS cohort, we identified 15 household characteristics. These household characteristics were indoor combustion activities, exposure to TRAP and ambient PM_2.5 levels, seasonal variation, housing construction type, ventilation, and cleaning practices. Although we controlled for different variables in the two study regions, always using an extractor fan during cooking was consistently associated with a lower concentration of EPFRs in the home.

The lower EPFR concentrations observed in homes with consistent extractor fan usage during cooking highlight the significant impact of household cooking practices on indoor air quality. Research on cooking fume by-products has found that toxic compounds such as polycyclic aromatic hydrocarbons (PAH), aldehydes, alkanoic acids, heterocyclic aromatic amines, and other harmful products can be released when cooking at higher temperatures (36–38). It is established in the literature that EPFR concentrations can be generated from solid fuel combustion used for cooking (14, 20), but a close co-existence of EPFRs and cooking fume by-products, particularly PAHs, has also been reported (39–41). Regardless of whether EPFR originates from the fuel source used in cooking or from heating the food at high temperatures, the reduction of cooking emissions through regular use of an extractor appears to lower this oxidant component in house dust. It should be noted that the range of the confidence interval obtained indicates considerable uncertainty regarding the concise magnitude and direction of effects. These wide intervals may be due to limited statistical power. Further investigations with larger populations would help to validate and refine these estimates.

A systematic review found that major indoor PM_2.5 sources were related to combustion activities, including cooking, smoking, wood stoves, and the use of candles and incense (42). Since studies have established the presence of EPFRs on PM surfaces, particularly PM_2.5 generated from combustion activities (6, 7, 11, 12, 14), the use of candles, incense, gas stoves/ovens or fireplaces in homes could potentially increase EPFR concentrations in house dust. Our results from the BIS cohort support this hypothesis, demonstrating higher EPFR concentrations for homes with a fireplace and increased use of candles or incense. However, our results found that gas ovens were associated with lower EPFR concentrations than electric ovens in the BIS cohort. This association was contrary to prior assumptions, but this may be explained by interactions between EPFRs and other pollutants released during gas combustion rather than being solely attributed to PMs. A systematic review reported that indoor gas cooking and heating are found to be major sources of indoor NO₂ (42), but the relationship between EPFRs and NO₂ is still not yet established. One study reported that EPFRs were positively correlated with vehicle emissions on highways, particularly elemental carbon (EC) and NO₂ (43). Another study supports the finding, demonstrating that PM_2.5 exposure to NO₂ increased EPFRs by 5–8 times, whereas PM_2.5 exposure to NO and ozone did not change EPFR signals (44). Conversely, an experimental study demonstrated that EPFR concentrations were reduced from interacting with NO₂ as EPFRs have the ability to donate electrons to form stable products (45). Additional research is needed to further understand the relationship between EPFRs and NO₂ and to assess the impact of gas ovens on indoor air quality.

Xu et al. measured EPFR concentrations in PM_2.5 released from various sources and found that PM_2.5 from vehicle exhaust emissions generated the highest amount of EPFRs (22). The concentration of EPFRs was also observed to differ between agricultural areas and residential areas, with lower EPFR signals reported in agricultural areas due to lower vehicle emissions (16). The results of the present study demonstrated that annual ambient PM_2.5 and neighborhoods with moderate to high traffic were associated with higher EPFR concentrations in the BIS cohort. Our findings on the importance of TRAP and ambient PM_2.5 in house dust further suggest that outdoor infiltration significantly contributes to indoor air pollution.

We found the season to be important in both settings. In ELLF, a cohort in a sub-tropical region, EPFRs were higher in winter compared with summer. In BIS, a cool temperate region, EPFRs were lower in spring than summer after controlling for a different set of variables. A limitation of this study is that we did not have samples collected in all seasons in ELLF – study visits were only conducted in summer and winter. This finding was inconsistent with the literature, as previous studies have shown seasonal differences. For example, studies conducted in China found that EPFR concentrations were higher in colder months, with a study estimating an equivalent EPFR exposure approximating 23–73 cigarettes per day during the cold periods (12, 13, 20, 21, 46). The authors attribute this to the widespread use of high central heating, especially coal combustion, during winter. In addition, studies have identified that EPFR concentration with an adjacent oxygen atom was more prominent in colder weather, suggesting incomplete combustion (12, 20, 46). Coal combustion is not a heat source in Brisbane, so more work is needed to understand the potential sources in this setting. Rainfall may also influence seasonal variation in EPFR concentrations. The concentration of EPFRs associated with airborne PM_2.5 in Beijing, China, was lower in the summer months, a result attributed to the clearing of PM_2.5 by precipitation (13). Brisbane typically experiences higher rainfall during summer (47), whereas Geelong experiences its highest rainfall in spring and lowest rainfall during summer (48). These seasonal differences in precipitation across locations may play a role in the seasonal variation reported in the current study.

In the present study, housing construction type, garage type, house age, and housing material were associated with EPFRs in house dust. A systematic review reported that ventilation, house age, and attached garages are important household characteristics that predict indoor air pollutants in Canadian homes (49). These household structures likely influence the overall airtightness of a home (50–52). The air exchange rate (AER) is typically used to understand the leakiness of a house by determining the rate of outdoor air replacing indoor air in a specified space, and AER is one of the main factors that determine indoor air quality (42, 53). Measured AER > 1 (exchange per hour) indicates that outdoor sources are more pronounced, and indoor PM concentrations tend to closely resemble outdoor levels due to indoor air being replaced with outdoor air (54). An Australian report found older houses (over 50 years) in Australia tended to have higher AER (above the 90^th percentile of 1.3 h⁻¹), signifying higher outdoor air penetration than newer houses (55). Further, houses built with weatherboard cladding (1.0 h⁻¹) have higher mean AER values than brick houses (0.5 h⁻¹) (55). Higher air penetration in timber-constructed houses may be due to an increased likelihood of structural degradation from termites, sunlight, and water damage (56). Furthermore, a study investigating air exchange rates (AERs) reported a mean AER of 0.44 h⁻¹ with windows closed and 1.57 h⁻¹ with windows open, highlighting the greater influence of outdoor air on the indoor environment when ventilation is increased (57). Our findings in the BIS cohort were consistent with those of (55), demonstrating higher EPFR concentrations of EPFRs in older homes and those constructed with weatherboard compared to brick. In the ELLF cohort, we found that higher natural ventilation and increased frequency of windows or doors opened in the ELLF cohort were associated with lower levels of EPFRs in house dust after controlling for ambient PM_2.5 concentrations. High AER can either improve or worsen indoor air quality, depending on the location of the households and the levels of outdoor air pollution. The difference in EPFR concentration associated with high AER observed across different household characteristics in Brisbane and Geelong can be attributed to the lower annual outdoor PM_2.5 levels in the ELLF cohort compared to the BIS cohort. Furthermore, the type of garage can influence indoor air quality, with previous research demonstrating that attached garages or underground parking garages led to higher concentrations of pollutants (volatile organic compounds, PM, CO, and NO₂) inside the houses when compared to non-attached garages (42, 51, 52, 58). Pollutants generated in enclosed and attached garages from the car can accumulate and penetrate inside the home through walls and ceilings (51, 52). Therefore, we hypothesized that enclosed garages would be associated with greater EPFR concentrations due to greater levels of pollutants. However, we found a negative association in the BIS cohort. The reason for the observed negative association remains unclear, but we believe that the interaction between NO₂ and EPFRs, as previously discussed, may contribute.

Finally, measures to maintain cleanliness in the house were found to be important household characteristics of EPFRs in Australian homes. Measures such as removing shoes before entering homes, using doormats, removing carpets, dusting with damp cloths, and regular cleaning can reduce dust concentration and indoor air pollutants (57). We hypothesize that these cleaning measures can potentially lower EPFRs generated from indoor combustion or infiltration of outdoor air pollution. Our results showed that only the ELLF cohort demonstrated higher EPFRs for households that do not practice regular cleaning and use a dry cloth or dusting wand to clean surfaces. The presence of fabric flooring, carpets, or rugs throughout the home was associated with greater EPFR concentrations in the BIS cohort but lower EPFR concentrations in the living area for the ELLF cohort. The inconsistencies in the direction of associations between settings, with the BIS cohort demonstrating higher EPFR concentrations with increased cleaning frequency and fabric flooring throughout the home, may be attributed to ineffective cleaning practices. In addition, our findings demonstrated that houses that had undergone a major renovation were associated with higher EPFR concentrations in the ELLF cohort. Research on increased small PM (aerodynamic diameter of 1.7 μm) from house renovations (59) and EPFRs has not been extensively studied. The current understanding is that EPFRs are generated through combustion (6, 10–12, 14). Therefore, more research is warranted to understand the importance and mechanism of increased PM from major construction and EPFR concentrations.

While the results demonstrated that indoor EPFRs can be generated through household activities, components of TRAP—particularly PM_2.5—also significantly influence EPFR concentrations within homes.

Our research provided insight into the potential impact of household characteristics on EPFR concentrations, which can potentially lead to adverse health effects. Future research should link our research findings on EPFRs—affected by factors such as the use of extractor fans during cooking, indoor combustion activities, exposure to TRAP and ambient PM2.5 levels, seasonal variation, housing construction type, ventilation, and cleaning practices—to their potential health effects.

This study has a few limitations that need to be acknowledged. Although the Barwon Infant Study (BIS) provided a valuable comparison cohort to the ELLF study, we were unable to age-match the children’s health outcomes, as dust samples in BIS were collected only at 9 months of age. As a result, our analysis focused solely on investigating household characteristics associated with EPFR concentrations in house dust across the two locations. Another limitation is the relatively small sample size, which constrained the statistical power of the study. Recruitment in both cohorts was limited by factors such as time, equipment availability, research staff, and funding limitations. Despite these challenges, the study also has several notable strengths. First, we anticipated that the small sample size in the ELLF cohort could lead to instability in model selection and wide confidence intervals. To address these concerns, we included data from the larger BIS cohort in Geelong. We also applied multiple approaches to evaluate the robustness of our results: (1) confidence intervals were estimated using both selective inference following lasso variable selection and standard linear models, and (2) we conducted a bootstrap analysis to assess the stability of results under resampling. These complementary methods allowed for a more comprehensive assessment of uncertainty (34). In addition, this prospective study reduced the risk of recall biases and provided more reliable information on exposures. Finally, the air pollution data for the ELLF cohort were obtained from monitors placed directly in participants’ homes, providing a more accurate representation of personal pollutant exposure compared to relying solely on publicly available environmental data.

This study found several household characteristics associated with EPFR levels in Australian homes, including the use of extractor fans during cooking, exposure to TRAP and ambient PM_2.5 levels, indoor combustion activities, seasonal variations, housing construction type, ventilation, and cleaning practices. Environmentally Persistent Free Radicals (EPFRs) are oxidant components of air pollution and may play a role in human health impacts of air pollution exposure. This study found EPFRs are present in house dust and that key factors influencing EPFR concentrations included the usage of extractor fans while cooking, traffic-related air pollution, ambient PM_2.5, indoor combustion activities, season, housing structures, ventilation, and cleaning habits. Notably, consistent use of extractor fans while cooking was associated with lower EPFR concentrations in household dust. These findings underscore the importance of household practices and environmental factors in determining indoor air quality and the persistence of pollutants within the home.