HEPA purifiers, air-quality monitors and water filters: the standards that matter and the claims that do not.
Overall evidence: Moderate17 products · 20 key studiesReviewed October 7, 2026Researched with AI assistance · Not reviewed by a clinician · Our method
The verdict
What we know about air purifiers & water filters
Clean air and water are some of the least glamorous and best-grounded wellness investments. The useful questions are concrete: what a purifier removes, whether it is sized for your room, and which certification a water filter actually holds. This guide explains the evidence and the standards.
Bottom line · Moderate
HEPA air purifiers and certified water filters reliably do the specific job they are tested for: pulling fine particles out of indoor air, or cutting named contaminants such as lead and PFAS from tap water. The health payoff for a typical household is smaller and less certain: a few points off systolic blood pressure in short trials, mixed results for asthma, and, in blinded trials, no drop in stomach illness from filtering tap water that was already well treated. Skip air cleaners that produce ozone, and ignore 'alkaline' or 'structured' water claims.
HEPA purifiers: cutting indoor particlesStrong
Sham-controlled trials consistently show portable HEPA purifiers lower indoor fine particles (PM2.5), often by about half or more, when the unit is sized for the room and left running.
Air purifiers and blood pressureModerate
Pooled sham-controlled trials show a small, short-term drop in systolic pressure of about 2-4 mmHg, clearest in people whose pressure is already elevated. Trials are short, certainty is limited, and effects on heart attacks or strokes are untested.
Air purifiers for asthma and allergiesWeak
Mixed. Large, well-run trials cut indoor pollution but did not reduce asthma symptoms; some studies show small gains in quality of life or allergy-medication use.
Wildfire smokeModerate
Portable HEPA cleaners cut indoor smoke particles substantially (54-92% in reviewed studies) and health agencies recommend them; trials measuring health outcomes during smoke events are still lacking.
Ozone generators and ionizers (risk)Strong
EPA finds ozone generators do not clean indoor air effectively at safe ozone levels, and both EPA and California regulators warn that ozone harms the lungs. Some ionizers and other electronic cleaners can also emit ozone.
CO2 monitors and thinkingWeak
Lab studies disagree on whether CO2 at everyday indoor levels dulls thinking; several found no effect from CO2 itself. A CO2 monitor is best treated as a gauge of how well a room is ventilated.
Water filters for leadStrong
Field studies show filters certified to NSF/ANSI 53 for lead reliably cut lead at the tap when used and maintained as directed.
Water filters for PFASModerate
Carbon, ion-exchange and reverse-osmosis units can greatly reduce PFAS in testing, but real-home data are limited and current certifications do not yet confirm reduction to EPA's 2024 limits.
Filtering already-safe tap waterWeak
Blinded trials in places with well-run water systems found no drop in stomach illness from home filters; one older unblinded trial on a sewage-affected supply did find a reduction.
Reverse osmosis trade-offsModerate
Reverse osmosis removes or reduces a wide range of contaminants, but it also reduces minerals and fluoride and sends water down the drain; whether the mineral loss matters for health is unproven.
Alkaline and 'structured' waterWeak
No good evidence of health benefits. A systematic review found no trials supporting alkaline water for cancer, and we found no human trials of 'structured' water.
Evidence
What the research shows
Strong
Portable HEPA air purifiers lower indoor fine-particle (PM2.5) pollution, often by about half or more.123Reductions depend on room size, fan speed, open windows and whether the unit is actually left on; across blood-pressure trials the average reduction was about 60% (range 23-82%).
Moderate
Running a HEPA purifier at home can lower systolic blood pressure by a few points in the short term.245Pooled effects are about 2-4 mmHg systolic with little change in diastolic pressure; trials lasted days to weeks, and one review rated the certainty 'very low.' A 2025 US trial found a net 3 mmHg benefit only in people whose systolic pressure was already 120 or higher. It is not a substitute for medical treatment of high blood pressure.
Weak
Air purifiers reduce asthma symptoms.167Large sham-controlled trials in classrooms (2021) and homes (2025) cut pollution without reducing symptom days. A review for US asthma guidelines found air purifiers improved quality of life only with low strength of evidence and did not improve validated asthma-control or lung-function measures.
Early
Air purifiers may reduce medication use for dust-mite allergic rhinitis.8One 6-week placebo-controlled trial; medication scores improved, but symptom and quality-of-life scores did not.
Strong
During wildfire smoke, a portable HEPA cleaner substantially cuts indoor smoke particles.3Reviewed studies found indoor PM2.5 reductions of 54-92%, while naturally ventilated homes were only 0-44% cleaner indoors than outdoors. This is exposure evidence; health outcomes during smoke events have not been tested in trials.
Strong
Ozone generators, and some ionizing or other 'electronic' purifiers, can raise indoor ozone to harmful levels.9Measured ozone from some ionization and ozone-generating purifiers exceeded EPA, CARB or OSHA safety levels in real rooms. Not every ionizer emits meaningful ozone; California-certified air cleaners must emit less than 0.050 ppm.
Weak
CO2 at levels common indoors (around 1,000 ppm) directly impairs thinking.101112One chamber study found decision-making declines at 1,000 ppm, but a later trial found no effect of pure CO2 up to 3,000 ppm, whereas stale air from people (bioeffluents) did cause headaches and slower performance. A high reading mainly signals poor ventilation.
Strong
Filters certified to NSF/ANSI 53 for lead reliably cut lead at the tap.13In field studies 99% of certified filters (mostly faucet-mounted, plus some pitcher and under-sink units) met the certification benchmark; failures were tied to very high lead levels, lead particles, or improper use and maintenance.
Moderate
Certified home filters (activated carbon, ion exchange or reverse osmosis) reduce PFAS 'forever chemicals.'14Lab testing and certification are encouraging, but few studies have measured PFAS at the tap in real homes; short-chain PFAS are harder to remove, and poorly maintained carbon filters may even raise PFAS levels. EPA notes current certifications do not yet show reduction to its 2024 limits.
Weak
A home filter prevents stomach illness when tap water is already well treated.151617Blinded trials with look-alike sham devices in Iowa and Melbourne found no reduction. An older trial without a sham device, in a community whose source water was sewage-affected, found less stomach illness in households given reverse-osmosis filters.
Weak
Removing minerals from drinking water, as reverse osmosis does, harms health.18Observational studies link more magnesium in drinking water with modestly lower coronary heart disease deaths (RR 0.89), but they cannot show cause and effect, results were inconsistent, and nearly all came from Europe.
No good evidence
Alkaline water prevents or treats disease.1920A systematic review found no randomized trials and no evidence supporting alkaline water for cancer. Other trials are small and short and measure lab markers (one found slightly lower blood viscosity after exercise), not health outcomes.
No good evidence
'Structured' or 'hexagonal' water improves health.Our PubMed search found no human clinical trials testing these products; the claims are not backed by clinical evidence.
Key studies
The studies behind the ratings
Summaries are our own, from each study's published abstract. Follow the links for the full details.
Systematic review and meta-analysis of true- vs sham-mode filtration studies · 2023
An updated systematic review and meta-analysis on portable air cleaners and blood pressure: Recommendations for users and manufacturers
Who:
17 studies (China, USA, Canada, South Korea, Denmark); about 880 participants
What:
Portable air cleaners run in true vs sham mode
Found:
Systolic BP -2.35 mmHg (95% CI -4.5 to -0.2); diastolic -0.81 mmHg (-1.86 to 0.24, not significant). Excluding high risk-of-bias studies: systolic -3.62 (-6.69 to -0.56) and diastolic -1.35 (-2.29 to -0.41). Air cleaners cut indoor PM2.5 by 59.8% on average (range 23-82%).
Limits:
Studies varied in design and pollution level (outside China, most were in relatively low-pollution settings); blood pressure is a surrogate, not heart attacks or strokes.
Systematic review and meta-analysis of randomized controlled trials · 2021
Effectiveness of indoor air purification intervention in improving cardiovascular health: A systematic review and meta-analysis of randomized controlled trials
Who:
14 crossover RCTs (18 publications), all ages
What:
Indoor air purification vs sham
Found:
Systolic BP -2.28 mmHg (95% CI -3.92 to -0.64); diastolic -0.35 mmHg (-1.52 to 0.83, not significant); C-reactive protein and vascular-function changes not significant overall. Overall GRADE certainty: very low.
Limits:
Small samples, short interventions and some missing washout periods; the authors caution against unsubstantiated cardiovascular claims.
Effect of HEPA Filtration Air Purifiers on Blood Pressure: A Pragmatic Randomized Crossover Trial
Who:
154 adults living next to highways (mean age 41.1; mostly higher socioeconomic status; mean baseline BP 118.8/76.5 mmHg)
What:
In-home HEPA air purifiers vs sham filtration
Found:
HEPA cut indoor particles vs sham. In people with systolic BP of 120 mmHg or more, systolic fell 2.8 mmHg with HEPA and rose 0.2 mmHg with sham, a net 3.0 mmHg difference (P = 0.04). No significant benefit for diastolic BP or for people with normal systolic BP.
Limits:
Benefit limited to a subgroup; one month per condition; relatively low-pollution setting; particles measured in only a subset of homes.
Systematic review (AHRQ methods) informing US asthma guidelines · 2018
Effectiveness of indoor allergen reduction in asthma management: A systematic review
Who:
59 randomized and 8 nonrandomized trials in people with asthma
What:
Indoor allergen-reduction measures including air purifiers, HEPA vacuums, mattress covers, pest control and pet removal
Found:
No intervention improved validated asthma-control measures or lung function. Air-purifier studies showed better quality of life (low strength of evidence). Single interventions were generally not associated with improvement; some multicomponent programs reduced exacerbations.
Limits:
Heterogeneity prevented meta-analysis; many small studies; evidence often inconclusive.
Factorial randomized clinical trial; classroom HEPA purifiers vs look-alike sham units (blinded) · 2021
Effect of School Integrated Pest Management or Classroom Air Filter Purifiers on Asthma Symptoms in Students With Active Asthma: A Randomized Clinical Trial
Who:
236 students with active asthma in 41 urban elementary schools in the Northeastern US
What:
Classroom HEPA purifiers (filters changed every 3 months) for a school year
Found:
Asthma symptom-days per 2 weeks were 1.6 with HEPA vs 1.8 with sham, not significantly different (incidence rate ratio 1.47, 95% CI 0.79-2.75). No intervention-related adverse events.
Limits:
Classroom-only filtration; low baseline symptom levels; home use may differ.
Randomized placebo-controlled trial (HEPA cleaners vs identical units without filters) · 2025
AIRWEIGHS: A Randomized Controlled Trial of Air Cleaners in Normal Weight and Overweight/Obese Children With Asthma
Who:
164 children with asthma (mean age 10.9; 85% Black; 59% overweight or obese)
What:
Portable HEPA air cleaners at home for 12 weeks
Found:
Compared with placebo, indoor ultrafine particles fell 64.0%, PM2.5 64.4%, PM10 54.7% and NO2 11.7%, but there was no improvement in the primary outcome (maximum symptom days) or any secondary asthma outcome.
Effects of Air Purifiers on Patients with Allergic Rhinitis: a Multicenter, Randomized, Double-Blind, and Placebo-Controlled Study
Who:
Adults with house dust mite-induced allergic rhinitis
What:
Two HEPA purifiers (living room and bedroom) vs mock-up units for 6 weeks
Found:
Medication scores improved significantly vs placebo; symptom, visual analog scale and quality-of-life scores did not differ. Bedroom PM2.5 fell by up to 51.8%.
Limits:
Short trial; main symptom scores did not improve; sample size not reported in the abstract.
Interventions for reducing exposure to air pollution from landscape fires in a changing environment: A systematic review
Who:
33 studies from four countries on exposure to landscape-fire (wildfire and prescribed-burn) smoke
What:
Portable air cleaners, building HVAC filters, DIY fan-filter units, masks, clean-air shelters and alerts
Found:
Portable HEPA air cleaners reduced indoor PM2.5 by 54-92%. Building filters rated MERV 5/8 cut smoke PM2.5 by 18-58%, and MERV 12/13 by up to 87%. N95/P2 masks removed up to 94% in single-pass tests. Naturally ventilated homes were 0-44% lower indoors than outdoors.
Limits:
Mostly exposure measurements rather than health outcomes; few studies of long smoke events or low- and middle-income settings.
Quantification of ozone levels in indoor environments generated by ionization and ozonolysis air purifiers
Who:
Several ionization and ozonolysis air purifiers, including wearable units, tested in offices, bathrooms, bedrooms and cars
What:
Ozone emitted by purifiers at 40-50% relative humidity
Found:
In many cases ozone concentrations were well above public or industrial safety levels set by EPA, the California Air Resources Board and OSHA; ozone from multiple devices added together.
Limits:
Specific devices tested for a 2006 study; California-certified models now must emit under 0.050 ppm.
Indoor CO(2) concentrations and cognitive function: A critical review
Who:
37 experimental studies using objective cognitive tests
What:
CO2 raised by adding pure CO2 or by reducing ventilation
Found:
In the stronger studies, pure CO2 at common indoor levels affected only high-level decision-making on one test battery (Strategic Management Simulation) in non-specialized groups; reduced ventilation (with other pollutants building up) slowed some functions but left accuracy unaffected. Results across studies were inconsistent.
Limits:
Studies varied widely in tests, designs, participants and exposure control.
Controlled chamber exposure study (blinded, balanced order) · 2012
Is CO2 an indoor pollutant? Direct effects of low-to-moderate CO2 concentrations on human decision-making performance
Who:
22 adults
What:
CO2 at 600, 1,000 and 2,500 ppm in 2.5-hour sessions, ventilation held constant
Found:
Compared with 600 ppm, 1,000 ppm produced moderate, significant declines in 6 of 9 decision-making scales, and 2,500 ppm large declines in 7 scales (focused-activity scores rose).
Limits:
Small sample, one test battery and single-day exposures; the authors said confirmation was needed.
Effects of exposure to carbon dioxide and bioeffluents on perceived air quality, self-assessed acute health symptoms, and cognitive performance
Who:
25 subjects
What:
Pure CO2 added to 1,000 or 3,000 ppm vs the same CO2 levels produced by people's own bioeffluents, 255 minutes per condition
Found:
Adding pure CO2 had no significant effect on perceived air quality, symptoms or cognitive performance. Bioeffluents with CO2 at 3,000 ppm worsened perceived air quality, increased headache, fatigue and sleepiness, and slowed some tasks.
A randomized, controlled trial of in-home drinking water intervention to reduce gastrointestinal illness
Who:
Households in an Iowa community with a well-run utility but microbiologically challenged source water: 227 households (646 people) started on the active device and 229 households (650 people) on the sham
What:
In-home device with a 1-micron absolute ceramic filter plus ultraviolet light vs identical sham
Found:
No reduction in highly credible gastrointestinal illness: relative rate (sham vs active) 0.98 (95% CI 0.86-1.10).
Limits:
One community; may not apply to systems with treatment failures or to private wells.
Randomized intervention trial without a sham device · 1991
A randomized trial to evaluate the risk of gastrointestinal disease due to consumption of drinking water meeting current microbiological standards
Who:
Households drinking tap water made from sewage-contaminated surface water that met water-quality standards
What:
Reverse-osmosis home filters (299 households) vs usual tap water without a filter (307 households) for 15 months
Found:
Annual gastrointestinal illness incidence was 0.50 with filters vs 0.76 without (p < 0.01); the authors estimated 35% of reported illness among tap-water drinkers was water-related and preventable.
Limits:
Participants knew their group, and illness was self-reported in diaries; older study of a sewage-affected source.
Reviewing performance of NSF/ANSI 53 certified water filters for lead removal
Who:
23 studies covering 1,486 faucet-mounted, 25 under-sink and 167 pitcher filters (1,528 in field studies, 150 in lab studies)
What:
Filters certified to NSF/ANSI 53 (post-2007) for lead
Found:
99% (1,512 of 1,528) of filters used in field studies reduced lead to or below the certification benchmark (10 µg/L before 2019; 5 µg/L after); 61% (91 of 150) did in lab studies, which often tested conditions beyond the certification.
Limits:
Performance varied by filter type, water source and starting lead level; failures were linked to very high lead, particulate lead and improper operation or maintenance.
Removal of perfluoroalkyl and polyfluoroalkyl substances from tap water by means of point-of-use treatment: A review
Who:
Published studies of point-of-use and point-of-entry PFAS treatment
What:
Home granular activated carbon, ion-exchange and reverse-osmosis devices
Found:
Devices use the same technologies as treatment plants and are often certified for PFOA and PFOS, but few peer-reviewed studies measured PFAS at the tap after home treatment (only five in real-life use); not all filters were effective, especially against short-chain PFAS, and improperly maintained carbon systems might raise PFAS levels.
Limits:
Inconsistent test methods; long-term performance uncertain.
Systematic review of the association between dietary acid load, alkaline water and cancer
Who:
8,278 citations screened; 1 cohort study met inclusion criteria
What:
Dietary acid load and alkaline water
Found:
No randomized trials were found and no studies of alkaline water for cancer treatment; the one included study found no association between dietary acid load and bladder cancer (OR 1.15, 95% CI 0.86-1.55). The authors concluded promoting alkaline water for cancer prevention or treatment is not justified.
Limits:
Almost no research exists either way; review limited to cancer.
Effect of electrolyzed high-pH alkaline water on blood viscosity in healthy adults
Who:
100 healthy adults after exercise-induced dehydration
What:
Electrolyzed high-pH (alkaline) water vs standard purified water for rehydration
Found:
High-shear blood viscosity fell 6.30% with alkaline water vs 3.36% with standard water (p = 0.03); plasma osmolality, bioimpedance and body-mass change did not differ.
Limits:
A 2-hour lab marker, not a health outcome; single study.
Avoid ozone generators sold as air purifiers. EPA says ozone can damage the lungs, cause chest pain, coughing, shortness of breath and throat irritation, and worsen asthma, while at levels that do not exceed health standards it does little to clean indoor air. No federal agency has approved these devices for occupied spaces.2122
Electronic air cleaners (ionizers, electrostatic precipitators, some UV and 'plasma' units) may emit ozone; measured ozone from some ionization and ozone-generating purifiers has exceeded EPA, CARB or OSHA safety levels in real rooms and cars. California-certified models must emit less than 0.050 ppm ozone.92223
Filters only work when maintained: EPA notes a dirty, overloaded air filter won't work well; CDC warns that water filters need upkeep to keep germs from growing in them; and a 2024 review found improperly maintained carbon filters might even raise PFAS levels.142224
A filter certified only to NSF/ANSI 42 (taste, odor, chlorine) is not certified to reduce health-related contaminants such as lead or PFAS; those claims fall under NSF/ANSI 53 or 58 and are listed contaminant by contaminant.252627
Reverse-osmosis systems can reduce fluoride along with minerals such as calcium and magnesium, which is worth knowing if you rely on fluoridated tap water.24
Air purifiers are not a treatment for asthma or high blood pressure: trials show small or inconsistent effects on these conditions.267
This is general information, not medical advice. If you have a heart condition, are pregnant, take medication or have any health concern, ask a clinician first.
Buying guide
Specs that actually matter
Clean Air Delivery Rate (CADR) matched to the room
EPA's sizing chart works out to roughly two-thirds of the floor area in square feet (e.g., 130 cfm for 200 sq ft), assuming 8-foot ceilings; use the tobacco-smoke CADR for fine particles, size up for higher ceilings, and note ratings are measured at top fan speed.22
True HEPA filter
By the definition EPA cites (from the Department of Energy), a HEPA filter removes at least 99.97% of airborne particles 0.3 microns in size.28
No ozone: CARB-certified
California certification limits ozone emissions to under 0.050 ppm; EPA advises avoiding air cleaners that intentionally produce ozone.23
Noise rating you can live with
A purifier only helps if it runs; EPA notes noise depends on fan speed and that about 50 dB is roughly a modern refrigerator.22
Replacement-filter cost and availability
All filters need regular replacement and an overloaded filter won't work well, so the running cost matters as much as the sticker price.22
ENERGY STAR label (efficiency only)
ENERGY STAR means the unit is energy efficient; it says nothing about how well it cleans air.22
CO2 monitor with a true (NDIR) CO2 sensor
Many cheap monitors estimate 'eCO2' from other gases; a direct sensor gives a more meaningful ventilation reading.
Use CO2 as a ventilation gauge
CDC's archived COVID-era guidance says CO2 monitoring can provide information on ventilation and used readings below 800 ppm as one target for good ventilation.29
Know your water first
Your utility's annual Consumer Confidence Report shows what has been found in your water, so you can buy for a real problem.30
Certification that matches the contaminant
Health-related claims such as lead, cyst and PFOA/PFOS reduction are certified under NSF/ANSI 53 and listed contaminant by contaminant; check the specific claim in NSF's listings rather than trusting a generic 'certified' badge.26
For PFAS: NSF/ANSI 53 or 58 with a PFAS claim
EPA says to look for certification specifically for PFAS reduction; a product can be certified to a standard for other contaminants only.27
Filter type: certified performance first
In field studies, 99% of certified lead filters (mostly faucet-mounted, plus some pitchers and under-sink units) met the benchmark, but performance varied by filter type and water conditions.13
Pitcher vs faucet vs under-sink vs countertop: convenience and cost
Pitchers cost least up front but filter slowly in small batches with frequent cartridge changes; faucet-mount units filter on demand; under-sink systems (carbon block or RO) handle more volume with fewer changes but cost more to install; countertop gravity or RO units need no plumbing but take counter space.
Reverse osmosis: WaterSense label
Typical RO units waste 5 or more gallons per gallon treated; WaterSense-labeled units use 2.3 gallons or less.31
Cartridge replacement schedule and cost
CDC says filters must be maintained to keep working and to keep germs from growing, and poorly maintained carbon filters may release PFAS.24
Compare prices
Air purifiers & water filters: current prices
How we make money: some shop links are affiliate links, so ThriveRadar may earn a commission if you buy, at no extra cost to you. It never changes our evidence ratings. Prices are checked daily, not live; confirm the final price at the shop. Learn more.
Long-form interviews and explainers from researchers and clinicians. Their views are their own; check claims against the studies above.
How to Optimize Your Water Quality & Intake for Health
Andrew Huberman
Huberman covers hydration needs and timing, claims about alkaline/"pH" and hydrogen-enriched water, how to test tap water for contaminants, and options for filtering drinking water.
UC Davis engineering dean and indoor-air researcher Richard Corsi shows how to build the low-cost Corsi-Rosenthal box air cleaner he co-devised from a box fan and furnace filters.
Time to clear the air in the room... and talk about indoor air quality | Joseph Allen | TEDxBoston
TEDx Talks
Healthy-buildings researcher Joseph Allen argues that indoor air is an overlooked driver of health and that building design, operation and new low-cost monitors can improve it.
Environmental pollution and longevity: air pollution, noise, light, EMFs, & more (AMA 87 sneak peek)
Peter Attia MD
AMA preview in which Peter Attia weighs the evidence on air, noise and light pollution and EMFs, separating well-supported risks from weaker claims and discussing practical mitigation.
Roger Seheult explains what "forever chemicals" such as PFAS, PFOA and PFOS, and the plastics chemical BPA, are and what is known about their health effects.
RTINGS' test lab compares a DIY Corsi-Rosenthal box with the 26 commercial air purifiers it has measured, explains its test method, and why the DIY unit came out on top.
Peter Attia reviews exposure routes and health evidence for microplastics, PFAS, BPA and phthalates and offers a framework for reducing exposure; the public feed carries a preview, with the full episode for members.
Environmental health scientist Ian Mudway assesses what current evidence shows about microplastic exposure and health, separating plausible concerns from alarm.
Science Vs talks with reproductive biologists and a paediatric endocrinologist about microplastics and endocrine-disrupting plastic chemicals, how much we actually ingest, and practical steps to cut exposure.
Revisits the case of Ella Kissi-Debrah, whose death was linked to air pollution, to explain why largely invisible fine-particle pollution harms health.
Weighs the evidence on community water fluoridation, from high-dose effects to recent studies on low-dose exposure and the brain, with an epidemiologist and a community-health scientist.
Latest research
New studies and news
Collected automatically from PubMed, safety recalls and science news. Not yet reviewed by us; inclusion is not an endorsement.
It will clean the air: HEPA units commonly cut indoor fine particles by about half or more. Health effects in trials are modest, with systolic blood pressure about 2-4 mmHg lower in short studies, while several large asthma trials found no symptom improvement. EPA describes the health improvements as typically small and not always noticeable.12622
What size air purifier do I need?
Match the Clean Air Delivery Rate (CADR) to the room. EPA's chart works out to roughly two-thirds of the floor area in square feet, for example about 130 cfm for a 200 sq ft room with 8-foot ceilings. Use the tobacco-smoke CADR for fine particles and size up for high ceilings, since ratings are measured at top fan speed.22
Are ionizers or 'ozone' purifiers safe?
Avoid anything that makes ozone on purpose: EPA says ozone harms the lungs and, at safe levels, does little to clean indoor air. Some ionizers and other electronic cleaners can also emit ozone, so choose a model certified by the California Air Resources Board, which caps ozone emissions at 0.050 ppm.92123
Do air purifiers help with wildfire smoke?
For exposure, yes: studies found portable HEPA cleaners cut indoor smoke particles by 54-92%, while naturally ventilated homes were only 0-44% lower indoors than outdoors. EPA recommends a right-sized unit that does not produce ozone, ideally in a 'clean room,' and says commercial cleaners with a known CADR are typically preferred over DIY box-fan filters. Trials of health outcomes during smoke events are still lacking.332
Is a CO2 monitor worth buying?
As a ventilation gauge, it can be useful: rising CO2 means a room is not getting much fresh air, and CDC's archived COVID-era guidance used readings below 800 ppm as one target for good ventilation. Claims that everyday CO2 levels hurt your thinking are unsettled; one lab study found effects at 1,000 ppm, while another found none from CO2 itself up to 3,000 ppm.10111229
Do I need a water filter if my water meets EPA standards?
Maybe not. CDC says that if your tap water has no harmful germs or chemicals you may not need a filter, and blinded trials in well-run water systems found home filters did not reduce stomach illness. A certified filter makes sense for a specific problem, such as lead from old plumbing or PFAS, and your utility's annual Consumer Confidence Report is the place to start.15162430
Which water-filter certification should I look for?
Match the certification to the contaminant: NSF/ANSI 42 covers taste, odor and chlorine; 53 covers health-related contaminants such as lead and PFOA/PFOS; 58 covers reverse-osmosis systems; and 401 covers emerging contaminants such as some pharmaceuticals and microplastics. For PFAS, EPA says to look for NSF/ANSI 53 or 58 certification specifically for PFAS reduction, and you can confirm claims in NSF's online listings.2526273334
Is reverse osmosis worth it?
Reverse osmosis removes or reduces a wide range of contaminants (CDC says it removes lead, copper and chromium and may reduce arsenic, fluoride and nitrate), and EPA found the RO units it studied can greatly reduce PFAS. The trade-offs are cost, maintenance, lost minerals and fluoride, and wastewater: typically 5 gallons or more per gallon treated, versus 2.3 or less for WaterSense-labeled units. Whether losing water minerals affects health is unproven.18242731
What happened with Berkey filters?
In March 2023 EPA issued a stop-sale order under federal pesticide law to a retailer of Berkey filters, saying claims that the filters remove pathogenic bacteria and viruses, together with silver used as an antimicrobial, made them pesticides that had to be registered. The action was about registration and labeling, not a finding about how well the filters remove contaminants.35
Is alkaline or 'structured' water healthier?
There is no good evidence that it is. A systematic review found no trials supporting alkaline water for preventing or treating cancer, other alkaline-water studies are small and look only at short-term lab markers, and we found no human trials of 'structured' water.1920
Sources
References
EPA: ozone generators sold as air cleaners are generally ineffective at concentrations that do not exceed public health standards, and are not effective at removing carbon monoxide or formaldehyde (page updated Feb 19, 2026). 21
California: since 2010, portable indoor air cleaners sold in California must be certified by the Air Resources Board to emit less than 0.050 ppm (50 ppb) ozone and to meet electrical safety standards; electronic in-duct air cleaners were added in October 2020. 23
CARB settled with Molekule in August 2019 for $149,000 over sales of uncertified indoor air cleaning devices in California; the company stopped selling the model until it was certified as meeting the 0.050 ppm ozone limit. 36
The National Advertising Division (BBB National Programs), acting on a Dyson challenge, recommended on Oct 25, 2019 that Molekule discontinue claims including quantified pollution-elimination claims, 'independent testing' references, superiority over HEPA, and asthma and allergy symptom-relief claims; Molekule agreed to drop the symptom claims and appealed other findings. 37
On appeal (Mar 26, 2020), the National Advertising Review Board found Molekule supported claims that its PECO filter can address bioaerosol and VOC pollution, but recommended it discontinue or modify unqualified pollution-elimination claims and stop claiming superiority over HEPA purifiers; Molekule said it would comply. 38
EPA's April 2024 drinking-water rule set enforceable limits of 4.0 parts per trillion each for PFOA and PFOS, 10 ppt each for PFHxS, PFNA and HFPO-DA (GenX), and a Hazard Index for mixtures, with initial monitoring due by 2027 and compliance by 2029. On May 18, 2026, EPA proposed keeping the PFOA/PFOS limits with an option for water systems to extend compliance to 2031, and proposed rescinding the PFHxS, PFNA, GenX and Hazard Index limits; both were proposals open for public comment, not final rules (page updated May 18, 2026). 39
EPA advises looking for certification to NSF/ANSI 53 or NSF/ANSI 58 specifically for PFAS reduction, and notes that current filter certification standards (as of April 2024) do not yet indicate removal down to EPA's new drinking-water limits; EPA found the carbon, ion-exchange and reverse-osmosis units it studied can greatly reduce PFAS. 27
In March 2023, EPA Region 8 issued a FIFRA stop-sale, use or removal order (docket FIFRA-08-2023-0014) to a Utah retailer of Berkey water filters, citing reason to believe Black Berkey filter products were unregistered and misbranded pesticides because of marketing claims about removing pathogenic bacteria and viruses and about using silver as an antimicrobial. The order concerned pesticide registration and labeling, not a test of filtering performance. 35
EPA WaterSense: a typical point-of-use reverse-osmosis system sends 5 gallons or more of reject water down the drain for each gallon treated (some up to 10); WaterSense-labeled systems must use 2.3 gallons or less. 31
NSF certification listings name the home water-treatment standards: NSF/ANSI 42 (aesthetic effects such as chlorine, taste and odor), 53 (health effects, e.g., lead, cysts, VOCs, PFOA/PFOS), 58 (reverse-osmosis systems) and 401 (emerging compounds and incidental contaminants such as certain pharmaceuticals, BPA and microplastics). NSF's PFAS (P473) listing shows PFOA/PFOS reduction claims evaluated under NSF/ANSI 53, with some products also listing 'Total PFAS' reduction. 40
EPA defines a HEPA filter (per the Department of Energy) as one that removes at least 99.97% of airborne particles 0.3 microns in size. 28
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EPA: ozone generators sold as air cleaners are generally ineffective at concentrations that do not exceed public health standards, and are not effective at removing carbon monoxide or formaldehyde (page updated Feb 19, 2026). epa.gov
California: since 2010, portable indoor air cleaners sold in California must be certified by the Air Resources Board to emit less than 0.050 ppm (50 ppb) ozone and to meet electrical safety standards; electronic in-duct air cleaners were added in October 2020. ww2.arb.ca.gov
EPA advises looking for certification to NSF/ANSI 53 or NSF/ANSI 58 specifically for PFAS reduction, and notes that current filter certification standards (as of April 2024) do not yet indicate removal down to EPA's new drinking-water limits; EPA found the carbon, ion-exchange and reverse-osmosis units it studied can greatly reduce PFAS. epa.gov
EPA defines a HEPA filter (per the Department of Energy) as one that removes at least 99.97% of airborne particles 0.3 microns in size. epa.gov
EPA WaterSense: a typical point-of-use reverse-osmosis system sends 5 gallons or more of reject water down the drain for each gallon treated (some up to 10); WaterSense-labeled systems must use 2.3 gallons or less. epa.gov
In March 2023, EPA Region 8 issued a FIFRA stop-sale, use or removal order (docket FIFRA-08-2023-0014) to a Utah retailer of Berkey water filters, citing reason to believe Black Berkey filter products were unregistered and misbranded pesticides because of marketing claims about removing pathogenic bacteria and viruses and about using silver as an antimicrobial. yosemite.epa.gov
CARB settled with Molekule in August 2019 for $149,000 over sales of uncertified indoor air cleaning devices in California; the company stopped selling the model until it was certified as meeting the 0.050 ppm ozone limit. ww2.arb.ca.gov
The National Advertising Division (BBB National Programs), acting on a Dyson challenge, recommended on Oct 25, 2019 that Molekule discontinue claims including quantified pollution-elimination claims, 'independent testing' references, superiority over HEPA, and asthma and allergy symptom-relief claims; Molekule agreed to drop the symptom claims and appealed other findings. bbbprograms.org
On appeal (Mar 26, 2020), the National Advertising Review Board found Molekule supported claims that its PECO filter can address bioaerosol and VOC pollution, but recommended it discontinue or modify unqualified pollution-elimination claims and stop claiming superiority over HEPA purifiers; Molekule said it would comply. bbbprograms.org
EPA's April 2024 drinking-water rule set enforceable limits of 4.0 parts per trillion each for PFOA and PFOS, 10 ppt each for PFHxS, PFNA and HFPO-DA (GenX), and a Hazard Index for mixtures, with initial monitoring due by 2027 and compliance by 2029. epa.gov
NSF certification listings name the home water-treatment standards: NSF/ANSI 42 (aesthetic effects such as chlorine, taste and odor), 53 (health effects, e.g., lead, cysts, VOCs, PFOA/PFOS), 58 (reverse-osmosis systems) and 401 (emerging compounds and incidental contaminants such as certain pharmaceuticals, BPA and microplastics). info.nsf.org
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