Filtration Efficiency of Air Conditioner Filters and Face Masks to Limit Exposure to Aerosolized Algal Toxins

Harmful algal blooms (HABs) can generate toxins that can be aerosolized and negatively impact human health through inhalation. HABs are often found in waterways near residences, therefore, aerosolized HAB toxins can potentially affect both indoor and outdoor air quality. Given that HABs are predicted to increase worldwide, effective mitigation strategies are needed to prevent the inhalation of aerosolized HAB toxins. In this work, we characterized both the particle filtration efficiency using particle sizing instruments as well as the mass concentration of different congeners of aerosolized microcystin (MC) toxins that penetrate through commercially available face masks and air conditioner (AC) filters. Particles were generated from cultures of the toxin-producing cyanobacteria Microcystis aeruginosa. Hydrophobic congeners of microcystin including MC-LF and MC-LW were enriched in aerosols compared to water, with MC-LR being the most abundant, which has implications for the toxicity of inhalable particles generated from HAB-contaminated waters. Particle transmission efficiencies and toxin filtration efficiencies scaled with the manufacturerprovided filter performance ratings. Up to 80% of small, microcystin-containing aerosols were transmitted through AC filters with low filter performance ratings. In contrast, both face masks as well as AC filters with high filter performance ratings efficiently removed toxin-containing particles to below limits of quantification. Our findings suggest that face masks and commercially available AC filters with high filtration efficiency ratings are suitable mitigation strategies to avoid indoor and outdoor air exposure to aerosolized HAB toxins. This work also has relevance for reducing airborne exposure to other HAB toxins, non-HAB toxins, pathogens, and viruses, including SARS-CoV-2, the virus responsible for the COVID-19 pandemic.


Additional Details of the Experimental Set Up and Toxin Analysis
. Description of Air Conditioning (AC) filters and face masks used for experimentation.

Corrections to the Particle Size Distribution
In order to combine the measurements made by the Scanning Mobility Particle Sizer (SMPS) and Aerodynamic Particle Sizer (APS), the electrical mobility diameter (dm) measured by the SMPS and the aerodynamic diameter (da) measured by the APS were both converted to the physical diameter (dp) using the following equations: 2 = (S1) where , , and are the physical, mobility, and aerodynamic diameters, respectively; is the effective particle density, and is unit density (1 g cm -3 ). We used an effective density of 1.8 g cm -3 for seawater 3 and an effective density of 1.5 g cm -3 for aerosols generated from M. aeruginosa-a value found to be appropriate for lake spray aerosol. 4 Because the SMPS tends to undercount particles in the upper size bins due to the impactor while the APS tends to undercount particles in the lower size bins due to poor scattering efficiency, the highest SMPS size bins and lowest APS size bins in the overlapping regions were discarded to combine data from the two sizing instruments. 2,3,5

Details of the Proof-of-Concept Experiments Using Filtered Seawater
Aerosols were generated by bubbling filtered seawater collected from the RSMAS dock. The bubbler apparatus is shown in Figure 1 and described in the main text. For the seawater experiments, we used a bubbler flow of 1.5 lpm and a dilution flow of 15.5 lpm. Flows were verified using a Sensidyne Gilibrator bubble flow calibrator. The bubbler and dilution flow were then mixed and passed through the filter cassette containing a 47 mm cutout of a face mask or AC filter. We also ran a control without a filter in this filter cassette. The flow that passes across the face mask or AC filter piece is then split using a four-way aerosol flow splitter (Brechtel Manufacturing Inc) to a filter cassette holding a 47 mm pre-combusted glass fiber filter (EPM2000), which was used to collect generated aerosols, a scanning mobility particle sizer (SMPS, model 3082, TSI Inc), and an aerodynamic particle sizer (APS, model 3221, TSI, Inc). For the seawater experiments, silica gel dryers were used upstream of the SMPS and APS and the relative humidity (RH) for the experiment was controlled to 60%, above the efflorescence RH of sea salts. 6 The fourth split was capped for this experiment.
The nine filter cut outs used were comprised of six commercially available air conditioner (AC) panel filters (labeled AF) and three types of personal face masks (labeled FM). Two modes of sampling were used: 1) without a mask or AC filter upstream, and 2) with a mask or AC filter upstream. The system was run without a mask or AC filter upstream (e.g., Mode 1) to collect the background sea spray aerosol particle size distribution for 30 minutes. Air filters were then placed in the experimental filter holder sequentially to measure the particle size distribution passing through the filter (Mode 2). Particle size data was continuously collected for at least 20 minutes for each experimental AC filter, and 60 minutes for each personal face mask. Fig S1 shows the size distributions of the average control run and the average run with each in-line filter. Table S2 shows the average composite particle filtration efficiency across all size bins for each filter cutout tested. Each particle size distribution is shown as a contour plot as a function of size and sample number for SMPS data in Fig S2 and

Size Distributions from Aerosol Generated from M. aeruginosa
Aerosols were generated by bubbling cultures of M. aeruginosa. We selected three filters representative of high and low filtration efficiencies for experiments with M. aeruginosa: AF4 (Honeywell FPR 10), AF5 (Rheem FRP 4), and FM2 (a disposable surgical mask). Two modes of sampling were used: 1) without a mask or AC filter upstream (Mode 1), and 2) with a mask or AC filter upstream (Mode 2). Particle size data was collected for at least 2 hours in each Mode for each experiment with M. aeruginosa. Fig. S4 shows a comparison of average size distributions collected in Mode 1 and Mode 2 for each filter tested. Each particle size distributions is shown as a contour plot as a function of size and sample number for data collected in Mode 1 in Fig S5 and