The Purification Encyclopedia

Water

What is in it, what treats it, and what the numbers mean.

Contaminants — 16 entries

• Total Dissolved Solids (TDS)
• Arsenic
• Fluoride
• Nitrate
• Lead
• Iron
• Total Hardness
• Chlorine
• Bacteria (E. coli & coliforms)
• Viruses
• Chloride & Salinity
• Calcium
• Magnesium
• Microplastics
• Turbidity
• Uranium

Technologies & categories — 16 entries

• RO purifiers
• UV purifiers
• UF purifiers
• Alkaline purifiers
• Commercial RO systems
• Industrial RO plants
• Water softeners
• Water filters
• RO membranes
• Sediment filters
• Carbon filters
• UV lamps
• TDS meters
• Water testing kits
• Water dispensers
• Water coolers

Specification terms

ppm and mg/L

Parts per million and milligrams per litre — numerically equivalent for dilute water solutions, so a result of 250 mg/L and 250 ppm are the same figure.

Commonly misread: The trap is not the unit but the basis. Nitrate reported 'as NO₃' and 'as NO₃-N' differ by a factor of about 4.43 for the same water, so a 45 mg/L result is either exactly at the BIS limit or four times over it depending on which line of the report you read.

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Rejection rate

The percentage of dissolved solids an RO membrane removes, calculated as (feed TDS − product TDS) ÷ feed TDS.

Measured against: Manufacturer test conditions — usually a standard NaCl solution at a specified pressure and temperature

Commonly misread: Rated rejection is measured on clean test water at full design pressure. Real rejection falls with membrane age, low feed pressure and high feed TDS, and it falls silently — output volume stays normal while treated-water TDS climbs. The only way to know your actual rejection is to measure feed and product TDS yourself.

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Recovery ratio

The share of feed water that leaves an RO system as product rather than as reject — 25% recovery means one litre produced for every three wasted.

Commonly misread: Domestic RO units are frequently sold on rejection rate while recovery goes unmentioned, and recovery is what shows up on your water bill. Recovery also falls as feed TDS rises, so the same unit wastes considerably more on a saline borewell than on municipal supply.

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Micron rating

The particle size a filter cartridge is designed to capture, stated either as nominal (most particles of that size) or absolute (essentially all of them).

Commonly misread: Nominal and absolute ratings are printed identically and mean very different things — a nominal 1 micron cartridge may pass a substantial share of 1 micron particles. Where the rating matters for a health claim, as with lead-reduction carbon blocks, only an absolute rating with a certification behind it means anything.

Log reduction

A logarithmic measure of how much a treatment reduces organism counts — 1-log is 90%, 3-log is 99.9%, 4-log is 99.99%.

Measured against: WHO expresses microbial treatment targets in log reduction rather than in finished-water concentration

Commonly misread: The gap between 99.9% and 99.99% looks trivial and is a factor of ten in surviving organisms. Because infectious doses for some enteric viruses are very low, that factor of ten can be the difference between adequate and inadequate treatment.

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NTUNTU

Nephelometric Turbidity Units — the standard measure of water cloudiness, determined by how much light a sample scatters.

Measured against: APHA 2130 B; BIS sets 1 NTU acceptable and 5 NTU permissible

Commonly misread: Turbidity is treated as a cosmetic parameter and it is not. The limit is tight because particles shield organisms from UV and chlorine — turbidity is a disinfection-performance parameter, and water above the limit means your UV unit may not be working regardless of what its indicator says.

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Grains per gallonGPG

A hardness unit used in softener sizing — one grain per gallon equals about 17.1 mg/L as calcium carbonate.

Commonly misread: Indian laboratories report hardness in mg/L as CaCO₃ while softener capacity is often quoted in grains, and a conversion error of 17× in either direction produces a badly sized unit. Confirm which unit a quotation uses before comparing two of them.

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NSF/ANSI certification

Third-party certification of a drinking water treatment unit against a specific standard — NSF/ANSI 53 for health-effect contaminant reduction, 58 for reverse osmosis systems, 42 for aesthetic effects.

Measured against: NSF/ANSI standards, verified by an accredited certification body

Commonly misread: 'Uses NSF-certified components' is not the same claim as 'certified to NSF/ANSI 58', and the difference is that the second one tested the finished product. For contaminants with a health basis — lead in particular — the certification is the entire claim, because there is no other way to verify a reduction percentage.

Acceptable vs permissible limit

BIS IS 10500:2012 states two figures for many parameters: an acceptable limit, and a permissible limit that applies only in the absence of an alternate source.

Measured against: IS 10500:2012, Bureau of Indian Standards

Commonly misread: The permissible figure is quoted as though it were the standard. It is a fallback for communities with no other supply — a statement that water at that level is tolerable when there is no choice, not that it is fine. Note also that some parameters, including nitrate and lead, have no permissible relaxation at all.

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Air

What is in it, what filters it, and where filtration is the wrong tool.

Pollutants — 13 entries

• PM2.5
• PM10
• Volatile Organic Compounds (VOCs)
• Formaldehyde
• Carbon Monoxide
• Ozone
• Nitrogen Dioxide
• Cooking Emissions
• Smoke
• Pollen
• Dust Mite Allergen
• Pet Dander
• Mould Spores

Technologies & categories — 12 entries

• Air purifiers
• HEPA filters
• Replacement filters
• Accessories
• Air quality monitors
• PM2.5 monitors
• Humidifiers
• Dehumidifiers
• UV air sanitizers
• Car air purifiers
• HVAC & AC air purification
• Industrial air purification

Specification terms

Clean Air Delivery RateCADR

The volume of particle-free air an air purifier delivers per hour, measured in cubic metres per hour (m³/h) or cubic feet per minute (CFM).

Measured against: ANSI/AHAM AC-1 test chamber method

Commonly misread: A CADR figure without a stated test standard is not comparable to one measured to AC-1, and manufacturers routinely publish the higher of the two. Worse, CADR is frequently translated into a 'coverage area' using two air changes per hour — which is barely enough to hold a room steady, let alone clean it during a pollution episode. Work from room volume and your own target air changes instead of trusting the square-footage claim on the box.

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Air Changes per HourACH

How many times per hour a purifier can process a volume of air equal to the whole room — calculated as CADR divided by room volume.

Commonly misread: Two air changes per hour is the figure most coverage claims are built on, and it is inadequate for Indian conditions. Four to five is the working target for a room you want measurably cleaner during a bad-air week. The difference roughly doubles the CADR you need for the same room, which is exactly why the lower figure is the one that gets printed.

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HEPAHEPA

A class of mechanical particle filter defined by minimum efficiency at its worst-case particle size — H13 requires at least 99.95% at the Most Penetrating Particle Size under EN 1822.

Measured against: EN 1822 (European classes E10–U17) or the US 99.97% at 0.3 µm specification

Commonly misread: 'HEPA-type', 'HEPA-like' and '99% HEPA' are not HEPA and are not covered by either standard — the words exist precisely to borrow the credibility without meeting the specification. Separately, HEPA is more efficient below 0.3 µm than at it, so the claim that HEPA 'cannot catch anything smaller than 0.3 microns' inverts the physics.

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Most Penetrating Particle SizeMPPS

The particle size a filter captures least efficiently — typically around 0.1–0.3 µm — where interception and diffusion capture mechanisms are both at their weakest.

Measured against: EN 1822, which rates filters at MPPS rather than at a fixed size

Commonly misread: People read 0.3 µm as the filter's lower limit. It is the filter's worst case. Capture efficiency rises again below MPPS because smaller particles move erratically and collide with fibres more readily. A filter rated 99.97% at 0.3 µm is typically better than that at 0.05 µm.

MERV ratingMERV

Minimum Efficiency Reporting Value — a 1 to 16 scale rating an HVAC filter's particle capture across three size bands.

Measured against: ASHRAE 52.2

Commonly misread: MERV and HEPA are different scales measured differently, so cross-converting them is approximate at best. Fitting a very high MERV filter to a duct system designed for a low one restricts airflow, which can reduce total particle removal even though the filter is better — the fan moves less air through it.

CARB certification

California Air Resources Board certification confirming an air cleaner emits under 0.050 ppm of ozone.

Measured against: California air cleaner regulation, tested to a defined chamber protocol

Commonly misread: It is an ozone emission limit, not a performance rating — a CARB-certified device is not thereby a good purifier, only one that is not adding ozone. It remains the single most useful third-party check available to an Indian buyer considering anything with an ioniser, plasma or photocatalytic stage.

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Air Quality IndexAQI

A single 0–500 index converting concentrations of several pollutants into one number, reported as the worst-performing pollutant at that moment.

Measured against: India's National AQI, published by CPCB, using eight pollutants

Commonly misread: AQI is a scale, not a unit, and different countries use different breakpoints — an AQI of 150 in India and in the US do not correspond to the same PM2.5 concentration. Because the index reports only the worst pollutant, a good AQI does not mean every pollutant is low, and an AQI figure cannot be converted back into µg/m³ without knowing which pollutant drove it.

Evidence and original research

Where this site publishes its own measurements rather than restating someone else’s.

• The Purifier LabTest protocols and measured results
• Filter autopsyWhat used filters actually look like
• Long-run diaries30-day and 6-month field logs
• Myth vs factClaims checked against physics and standards
• Filter price indexWhat consumables cost across brands
• Water quality by citySourced regional water characteristics
• Error code databaseSymptom → cause → what to check
• Don't buy thisWhen the answer is to spend nothing