Research Library

Wildfire

Wildfire Smoke Particles Are Mostly Smaller Than 0.3 Microns, Below What Standard Filter Tests Measure

Researchers at Metalmark Innovations tested seven HVAC filter media rated MERV 11 to 15 against pine-needle smoke, a stand-in for wildfire smoke. Most wildfire smoke particles are smaller than 0.3 µm, which standard filter tests do not measure. Fiberglass media removed smoke less efficiently than their MERV rating suggested: about 30% for MERV 11–14 and about 80% for MERV 15. Charged (electret) media started higher, but their efficiency dropped sharply as smoke built up. The paper also notes that wildfires emit hundreds of VOCs that infiltrate indoors. The study was carried out by a filter manufacturer.

Shirman, Shirman & Liu, Atmosphere 14(12):1729 (2023)

Wildfire Smoke Lowers Students' Test Scores, Even at Low Levels

Stanford researchers Jeff Wen and Marshall Burke matched about eight years of standardized test scores from nearly 11,700 U.S. school districts with satellite estimates of daily smoke exposure. English and math scores fell even at low smoke levels, and the losses grew with heavier exposure. Heavy smoke during the school day had nearly twice the effect of heavy smoke on weekends, and students in grades 3–5 were affected more than those in grades 6–8. The authors estimate that smoke exposure in 2016 alone could lower U.S. students' cumulative future earnings by nearly $1.9 billion.

Wen & Burke, Nature Sustainability 5:947–955 (2022)

Wildfire Smoke Contributes to Nearly 16,000 U.S. Deaths a Year

A National Bureau of Economic Research working paper led by Stanford researchers linked wildfire smoke exposure to all recorded U.S. deaths from 2006 to 2019. It attributes nearly 16,000 deaths a year to wildfire smoke, and projects about 27,800 excess deaths a year by 2050 under a high-warming scenario, with annual damages of $244 billion. Because smoke travels hundreds of miles, the risk reaches far beyond fire zones.

Qiu et al., NBER Working Paper 32307 (2024)

Mold & Mildew

EPA: Keep Humidity Below 60% and Dry Water Damage Within 24–48 Hours

The U.S. Environmental Protection Agency advises keeping indoor relative humidity below 60%, ideally between 30% and 50%. It also advises drying water-damaged areas and items within 24–48 hours: when wet or damp materials are dried within that window, in most cases mold will not grow.

U.S. EPA, A Brief Guide to Mold, Moisture and Your Home

National Academies: Damp, Moldy Homes Are Linked to Coughing, Wheezing and Asthma Symptoms

An Institute of Medicine committee, sponsored by the CDC, reviewed the evidence on indoor dampness and health. It found sufficient evidence that mold and damp conditions are associated with asthma symptoms in people with asthma who are sensitive to mold, and with coughing, wheezing and upper respiratory symptoms in otherwise healthy people. The committee said the evidence did not establish a causal relationship, and found it too limited to link mold to a wider range of illnesses. It noted that promptly fixing leaks and cleaning up spills or standing water substantially reduces the potential for mold growth.

Institute of Medicine, Damp Indoor Spaces and Health (2004)

Peer-Reviewed Study: WellisAir Reduced Airborne Molds and Yeasts in a Working Dental Clinic

Dental researchers in Barcelona ran a WellisAir WADU-02 hydroxyl unit in a 10.9 m² dental office at Centro Médico Teknon over five days in 2021, with routine cleaning continuing throughout. Compared with control days, airborne molds and yeasts fell by 66.7% (p = 0.03). No molds or yeasts grew on surface samples on either control or test days, so no surface reduction could be measured. The authors called for larger, controlled clinical studies.

Paños-Crespo, Toledano-Serrabona, Sánchez-Garcés & Gay-Escoda, Medicina Oral, Patología Oral y Cirugía Bucal 29(1):e145–e151 (2024)

Volatile Organic Compounds (VOCs)

Hydroxyl Radicals Are the Atmosphere's Natural Cleaning Mechanism

Atmospheric chemists Jos Lelieveld and colleagues describe how hydroxyl (OH) radicals start the photochemical reactions that oxidize most gases in the lower atmosphere, turning them into products that rain and surface deposition remove more easily. Through this self-cleansing process, thousands of megatons of natural and human-made gases are removed from the troposphere each year.

Lelieveld et al., Atmospheric Chemistry and Physics 4:2337–2344 (2004)

Pulmonary Health Effects of Indoor VOCs: A Meta-Analysis

A meta-analysis of 49 studies found that indoor VOC exposure has a medium-sized effect on lung health, including the onset of asthma (effect size about 0.37) and wheezing (about 0.26). Most of the studies came from high-income countries, where people spend much of their time indoors.

Alford & Kumar, Int. J. Environ. Res. Public Health 18(4):1578 (2021)

A Rapid Review of the Impact of Increasing Airtightness on Indoor Air Quality

A review of 20 studies found limited evidence of direct links between building airtightness and indoor air quality overall. Tighter buildings tended to have higher CO2 levels, and there was some evidence of higher VOC and formaldehyde levels, though several studies found no clear relationship. Where outdoor pollution is high, airtightness reduced the entry of outdoor contaminants. In short, airtightness tends to help against pollution from outside and worsen pollution from sources inside the home.

Kempton, Daly, Kokogiannakis & Dewsbury, Journal of Building Engineering 57:104798 (2022)

EPA: Many VOCs Are Higher Indoors Than Outdoors

The EPA reports that concentrations of many VOCs are consistently higher indoors, up to ten times higher, than outdoors. Its TEAM study found about a dozen common organic pollutants at 2 to 5 times higher levels inside homes than outside, whether the homes were in rural or highly industrial areas. Sources include paints, solvents, cleaners, disinfectants, air fresheners, building materials and furnishings. Benzene, one indoor VOC, is a known human carcinogen.

U.S. EPA, Volatile Organic Compounds' Impact on Indoor Air Quality

Compare to HEPA

Bacteria Can Grow on HVAC Air Filters in Humid Conditions

A 15-month field study of F6 and F7 filters in two HVAC systems found that, in dry and warm conditions, the filters cut airborne bacteria by about 70% and molds by more than 80%. During long periods of relative humidity above 80%, bacteria multiplied on the filters and were released into the filtered air; most were smaller than 1.1 µm, small enough to reach deep into the lungs. The study covered building HVAC filters, not portable air purifiers.

Möritz, Peters, Nipko & Rüden, Int. J. Hyg. Environ. Health 203(5–6):401–409 (2001)

EPA: Air Cleaner Ratings Cover Particles Only, and Every Filter Needs Replacing

EPA's guide to air cleaners explains that the Clean Air Delivery Rate (CADR) rating system is for particles only, and that most filters are designed to filter either particles or gases. To filter gases, EPA recommends an air cleaner with activated carbon or another filter designed for gases, and notes there are no widely used performance ratings for gas filters. All filters need regular replacement: a dirty, overloaded filter won't work well.

U.S. EPA, Guide to Air Cleaners in the Home

CDC Survey: Cost and Perceived Need Keep Many Households From Using Air Cleaners

In an August 2023 CDC survey of 7,599 U.S. adults, those who don't use an air cleaner at home most often said they didn't think one was needed (66%) or that it was too expensive (59%); 30% said air cleaners are too noisy. Among adults who use an air cleaner without a HEPA filter, 46% said HEPA units are too expensive and 30% had never heard of them.

CDC National Center for Health Statistics, Rapid Surveys System, Round 1: Air Cleaners and Purifiers (2023)