Brent Stephens

Evaporative coolers (ECs) introduce outdoor air pollutants indoors when operating. This study evaluates the potential of do-it-yourself (DIY) air filtration solutions for ECs to cost-effectively reduce the infiltration of ambient fine particulate matter (PM_2.5) in homes with ECs using measurements in 48 homes in wildfire-affected agricultural communities in California. All homes received one portable air cleaner (PAC); 25 homes also received DIY filters (mostly MERV 13) attached to their ECs. PurpleAir monitors measured indoor and outdoor PM_2.5 concentrations. PAC operation was monitored in all of the homes. EC usage was monitored in some homes and predicted using relative humidity dynamics in all homes. Conditional analyses between EC likely on and off conditions were used to evaluate the impacts of DIY EC filters on ambient PM_2.5 infiltration, including during several wildfire-affected days. Median levels of ambient PM_2.5 infiltration increased ∼36–42% in homes with only PAC interventions when ECs were likely operating compared to only ∼10–11% in homes with both PACs and DIY EC filters. Pre/postintervention comparisons in a subset of homes confirmed PM_2.5 infiltration reductions. EC filter performance declined after extended use. Results suggest that short-term EC filter deployments are likely a cost-effective way to mitigate wildfire smoke infiltration inside these homes.

Evaporative ("swamp") coolers (ECs) are used for cooling of homes in hot and arid climates. Because ECs draw in large volumes of outdoor air, they can introduce substantial amounts of ambient airborne particulate matter (PM) and other pollutants indoors, which is especially exacerbated during wildfire smoke events. We measured airborne metals and PM inside and outside farmworkers' homes with ECs in the California San Joaquin Valley, U.S.A.. Sampling was conducted as part of a larger study (FRESSCA, Filtration for Respiratory Exposure to wildfire Smoke from Swamp Cooler Air) to develop and evaluate an affordable EC filtration solution. All homes received portable air cleaners (PACs), and approximately half the homes received filters on their EC air intakes. Indoor and outdoor airborne metals were actively measured using 24-hr air sampling. Airborne metals were dominated by Fe, with some detections of Cu, Mn.Zn, and Se. Indoor/outdoor (I/O) ratios for Fe were mostly < 1 and were lower for homes with EC filters. Metals analyses of the EC filters themselves revealed additional metals, likely due to the longer sampling time. These metals were partially removed from the air entering participants' homes by EC filters. Passive PM samplers were deployed for 70 days and analyzed using electron microscopy. PM samples revealed crustal, carbonaceous, and agriculture-related particle types, including Cu-rich fungicides. PM levels were lower indoors compared to outdoors in all homes (30%-70%). Some evidence of greater PM reductions in homes with EC filters was observed, with 25% reductions in coarse PM_10-2.5 in homes with EC filters that used their ECs more frequently (p < 0.05). Few smoke events during the study limited our ability to detect differences in PM from wildfires. However, the measured reductions in coarse PM are important for communities heavily impacted by dust, both for home cleanliness and for protection from upper-airway health impacts.Implications Do-it-yourself evaporative cooler filters are a promising and affordable method of reducing dust infiltration in homes burdened by heat stress and poor outdoor air quality. Further evaluation during wildfire smoke events and optimization of evaporative cooler filter interventions could eventually reduce respiratory-related health impacts in communities affected by a changing climate.

Low-cost particle monitors have been widely evaluated in laboratory and ambient monitoring settings, but we have less knowledge about their performance for long-term indoor and outdoor monitoring in residential settings. We seek to provide practical guidance for using a type of low-cost particle monitors that have become widespread for indoor and outdoor monitoring in community field studies, PurpleAir PA-II monitors. We base our insights on experiences in a community-led residential field study in and around homes of predominantly agricultural workers in California’s San Joaquin Valley. Our guidance spans three categories: (1) providing tools for handling and merging disparate data structures resulting from Wi-Fi-transmitted data and data collected on onboard microSD cards, (2) assessing performance metrics of PA-II monitors from laboratory co-location and field measurements, and (3) assessing data collection success rates of Wi-Fi data transmission and microSD card data acquisition from our study locations. The post-processing methods we demonstrate can successfully align data from both Wi-Fi transmission and microSD cards. Laboratory co-location measurements demonstrated that > 90% of the tested monitors performed well relative to each other (high precision), with only a few problematic monitors that warranted further investigation or exclusion from use. The application of co-location factors generated using the mean of all co-located monitors as a reference did not significantly affect distributions of field-measured indoor or outdoor PM_2.5 concentrations. Relying solely on Wi-Fi data transmission in our study would have resulted in large data loss (i.e., < 50% success rate); using microSD card storage with PA-II-SD monitors increased the data collection success rate to over 80% in these settings. This work contributes to the growing body of knowledge on low-cost particle sensor performance and usability. Graphical abstract

Although people spend nearly 90% of their time indoors, the source-specific oxidative potential (OP) from indoor emissions spanning multiple particle generation mechanisms is largely unknown. Here, we quantify the OP of the PM originating from commonly used indoor sources via three different assays, i.e., dithiothreitol consumption (OP_DTT), glutathione consumption (OP_GSH), and hydroxyl radical generation (OP_OH). The intrinsic (mass-normalized) OP^m of several sources (candles, incense, cigarettes, humidifiers using tap water, toasters, and air fryers) were comparable (0.75-1.25×) or exceeded (1-6×) that of typical ambient PM_2.5. Emissions from candles in the presence of a wind-draft had the highest OP_DTT^m and OP_OH^m while OP_GSH^m was highest for cigarette emissions. Elemental carbon determined the OP of PM emitted from sources undergoing incomplete combustion, whereas, for noncombustion sources (water evaporation and heating-based), metals were the important drivers. An exploratory exposure assessment in a hypothetical apartment (volume = 100 m³, air change rate = 0.45 h^-1, and penetration coefficient = 0.6) revealed that certain sources (e.g., incense, cigarettes, toasters, and air-fryers) could result in occupants being exposed to higher OP in less than 1 h of indoor operation than that resulting from inhaling typical U.S. ambient PM_2.5 (8 μg/m³) over an entire day. Collectively, these results demonstrate the importance of indoor emissions, emphasizing the need for more comprehensive health impact assessments to assist in the development of policy recommendations aimed at mitigating indoor PM exposure.