Home Air & Water
Air purifiers, furnace filters, humidity and drinking water. What these four have in common is that every rating is defined by a published standard — and most of the confusing marketing in these categories dissolves once you know which standard applies and what it actually requires.
Four categories
Air Purifiers
CADR, HEPA grades and room sizing
HVAC & Furnace Filters
MERV ratings, depth and airflow
Humidifiers & Dehumidifiers
Output, dew point and mold risk
Water Filtration
NSF/ANSI certifications by contaminant
Which standard defines what
| Standard | What it defines |
|---|---|
| ANSI/AHAM AC-1 | How CADR is measured for portable air cleaners, and the 450 ceiling on reportable values |
| ANSI/ASHRAE 52.2 | MERV ratings, and the three particle bands each rating is tested against |
| US DOE (HEPA) | The 99.97% at 0.3 micron threshold, and why 0.3 is the hardest size |
| NSF/ANSI 42, 53, 58, 401 | What a water filter is certified to reduce — aesthetic, health, RO and emerging compounds |
| 10 CFR Part 430 App. X1 | The 65 °F / 60% test condition behind every portable dehumidifier pint rating |
| EPA & CARB guidance | Indoor humidity targets, filtration priorities, and the 0.050 ppm ozone limit on air cleaners |
Air purifiers
Sizing is the whole decision. Get CADR against your room right and a modest unit outperforms an expensive one; get it wrong and the machine runs all night doing very little.
- How to choose an air purifier — the full guide, with the two-thirds rule and what not to pay for
- What size do you need? — sizing from room volume, with four worked examples
- CADR explained — the three numbers, and why none exceeds 450
- True HEPA vs HEPA-type — the gap between 99% and 99.97%
- Activated carbon filters — the stage CADR does not rate
- Ionizers, UV and ozone — what the EPA and CARB actually say
HVAC and furnace filters
The EPA recommends at least MERV 13 — or the highest rating your fan and filter slot can take. The second half of that sentence is where most people go wrong.
- MERV ratings explained — the full EPA table and how to find your system’s ceiling
- 1-inch vs 4-inch filters — why depth beats rating
- How often to change it — six factors, and one check that beats any schedule
- MERV vs MPR vs FPR — why no conversion exists
Humidifiers and dehumidifiers
Both are governed by one number: relative humidity, which the EPA puts between 30 and 50 percent with a hard ceiling below 60.
- What indoor humidity to aim for — the dew point arithmetic behind the 60% ceiling
- How to choose a humidifier — output vs tank size, and the white dust problem
- Evaporative vs ultrasonic vs steam — three mechanisms, three consequences
- How to size a dehumidifier — why the pint rating changed
Water filtration
Filters are certified for named contaminants, so the decision runs one way only: find out what is in your water, then buy the certification that covers it.
- How to choose a water filter — start from your own water report
- NSF/ANSI 42 vs 53 vs 401 — which certification covers what
- Pitcher, faucet, under-sink or RO — choosing by contaminant
- Reverse osmosis — what NSF/ANSI 58 covers and what it costs you
The other half of the site
The kitchen cluster works the same way, but from materials and mechanism rather than ratings — how a metal moves heat, how a steel holds an edge, how a machine actually works.