Every shop air filtration system gets a score from 0 to 100% built from five factors, which maps onto the 5-point rating shown on the site (a 0% score would be 1.5 stars; a perfect score, 5). Each factor is scored against fixed reference points, not against the other products in the list, so adding a new model never moves another model’s rating. Units are grouped by type (ceiling-hung and portable), and the same scales apply to both.
| Factor | Measures | Weight |
|---|---|---|
| Value | price divided by top-speed airflow | 25% |
| Airflow | airflow at top speed | 25% |
| Filtration | finest particle size the inner filter is rated for | 20% |
| Features | remote, timer, three or more speeds, air-quality sensor and work light | 20% |
| Warranty | warranty length | 10% |
Where the numbers come from
Airflow, filter ratings, speeds, timers, noise, weight and warranty come from the maker’s product page where it publishes one, or otherwise from the listing copy at Amazon, which for most makers is the maker’s own text. Prices are checked at Amazon and other retailers, and the lowest price at a store the page links to is used. Prices are for the unit with its filters and mounting hardware, not for spare filters.
The factors
Value is the price divided by the airflow at top speed, on a log scale, so halving the price per CFM always counts the same. $1.00 per CFM scores zero and $0.12 per CFM full marks.
Airflow is the airflow at top speed in CFM: 300 CFM scores zero and 1,400 CFM full marks. This is free-air airflow at the fan, and makers quote it without a filter loaded, so a loaded unit moves less. It is not the CADR rating on a home air purifier, and the two can’t be compared.
Filtration is the finest particle size, in microns, that the listing rates the inner filter for, on a log scale: 5 microns scores zero and 0.3 micron (HEPA-class) full marks. Most units list a 1 micron inner filter, which scores about 57%. A listing that states a MERV rating without a micron figure is scored on the particle figure the listing gives.
Features add up what saves time: a remote control 30%, a timer 25%, three or more fan speeds 25%, an air-quality sensor that sets the speed itself 10% and a built-in work light 10%.
Warranty is the maker’s warranty: none scores zero and 5 years full marks, so 2 years scores 40%.
What isn’t scored
Noise, weight, the room size a maker claims to cover, a built-in ionizer and the cost of replacement filters are listed or noted but not scored. Ionizers are left out because their benefit isn’t established and some units that include one can’t be sold in California. Filter cost and life matter over time, but few listings publish them.
Missing data
If a figure isn’t published, that factor gets the neutral middle score (50) and the product page marks it as estimated. Features are neutral only when none of them is published; otherwise a feature the maker doesn’t mention counts as missing from the score. The exception is warranty: a maker that doesn’t publish one gets no warranty counted, so that factor scores zero and is marked estimated. Most listings, especially for lesser-known brands, publish no warranty, which is why unfamiliar brands can still rank high on value and airflow.
Ties and discontinued models
Products are ranked by score within each group. Discontinued models stay listed but rank below every current model and can be hidden with the “Hide discontinued” filter.