Impact of Wildfire Smoke Events on Indoor Air Quality and Evaluation of a Low-cost Filtration Method

Abstract

Increased wildland fire activity is producing extreme fine particulate matter (PM2.5) concentrations impacting millions of people every year, especially in the western United States (US). Recommendations for limiting exposure to PM2.5 and associated adverse health outcomes focus on staying inside, closing windows and doors, and increasing filtration; however, relatively little is known about indoor air quality (IAQ) during major smoke events. Indoor and outdoor hourly PM2.5 (μg m -3 ) measurements from the publicly available PurpleAir sensor (PAS) network 19 were analyzed for 42 sites (26 residential, 6 school, 10 commercial) across the western US during 20 a September 2020 period of heavy wildfire smoke influence. The fraction of ambient PM2.5 that penetrates indoors and remains airborne (Fin), as well as the ratio (I/O) and correlation coefficient 22 (R 2 ) of indoor to outdoor PM2.5 concentrations, were lower in residential compared to commercial 23 and school buildings. Interventions to improve IAQ were highly influential in PM2.5 infiltration in 24 residential case studies, with multiple, continuously run filter units associated with lower Fin, I/O, 25 and R 2 . A low-cost PM2.5 filtration method consisting of a Minimum Efficiency Rating Value-13 (MERV-13) filter attached to a box fan is evaluated as an alternative for improving IAQ during wildland fire smoke events. The MERV-13 fan filter unit proved highly effective at reducing indoor PM2.5 and particles 0.3 -1.0 μm measured by PAS and a particle counter, respectively, 29 when recirculating air in a single room. Low-cost filtration methods can have significant benefit for filtering submicron smoke particles and may reduce exposure to PM2.5 during wildfire smoke events.

Can 10x cheaper, lower-efficiency particulate air filters and box fans complement High-Efficiency Particulate Air (HEPA) purifiers to help control the COVID-19 pandemic?

“…Even though MERV-13 filters may remove less than 80% of particles in just one pass, if the air recirculates multiple times through filter the aerosol concentrations can be rapidly reduced. 7 The filtration efficiency exceeded 90% in some cases at particle sizes above 1 μm.…”

“…There is even an observable difference between the two Coway models as shown in Table 1. 7 The filtration efficiency exceeded 90% in some cases at particle sizes above 1 μm.…”

“…Lower cost alternatives to HEPA air purifiers have been in use for at least a decade and investigated for both their air filtration properties and fire safety. For example, a Minimum Efficiency Rating Value-13 (MERV-13) filter attached to a box fan using duct tape (or a bungee cord) has been demonstrated to effectively filter wildfire PM2.5 and submicron particles [12]. Ford Motor Company sponsored a project called "Scrappy Filtration" which reportedly donated 20,000 box-fan and air filters held together with a cardboard structure for use in classrooms to underserved communities [20] [21].…”

“…using duct tape, bungee cord, rubber band, clips) was demonstrated to effectively filter wildfire PM2.5 and submicron particles. 7 Ford Motor Company sponsored “Scrappy Filtration” which donated 20,000 box-fan and air filters held together with a cardboard structure for use in classrooms to underserved communities. 8 9 However, the published literature regarding these lower-cost box-fan-filters is lacking key metrics such as filtration efficiency (%), air flow (CADR), and noise generation (dBA) and does not use lower-cost, rapid test methods (e.g.…”

“…As shown in Table2, the filtration efficiency of the MERV-13, 14, 15, or 16 filters at the most penetrating particle size of 0.3 μm varied by thickness from approximately 60% to 85% with highest observed for MERV-16 filter and lowest for the MERV 13 filter, well below the HEPA filters. Even though MERV-13 filters may remove less than 80% of particles in just one pass, if the air recirculates multiple times through filter the aerosol concentrations can be rapidly reduced 7.…”