The Room Air Purifier Puff Up

Size does matter

The size of the room is important when choosing the best air purifier: for best purification of air, you need the power to deliver many air exchanges per hour. Air purifiers are marketed with a variety of room size ratings and claims, mostly misleading.

It is very important to make a realistic assessment of your space before purchase.

The home with heavy foot traffic across wall to wall carpet, three long haired cats who sleep on the bed, and combustion appliances or smoking, will be a challenge for even the best air cleaner.

A room size rating is only useful for comparing product air flow among very similar units.

We must ask: what is the air pollutant reintroduction rate?

Tests and ratings often do not measure real world conditions.

In fact, indoor air pollution is sporadic; study graphs record a series of peaks and valleys in particulate, odor, and VOC chemical levels.

Just walking and flopping down on the couch raises an invisible mushroom cloud of fine particles.

My Sharp FPN60CX dust sensor changes from green to orange when my wife quietly gets out of bed each morning.

When heat or A/C comes on more frequently, particle storms ensue.

Each particle peak may exceed the air purifier rating on high speed.

The Association of Home Appliance Manufacturers (AHAM) air purifier room rating (size in square feet) is based on the clean air delivery rate (CADR) test value for smoke, multiplied by a constant, 1.55.

CADR is rated with air purifiers on highest speed, with no reintroduction.

Basing a size choice on the assumption of 24/7 high speed purification is unrealistic. Many vendors further inflate the manufacturer’s or AHAM certified claims.

Rooms differ in shape, number and extent of obstructions, convection currents, and HVAC vent locations.

Air cleaners with equal airflow rates may differ in their ability to purify distant areas.

If the machine's exhaust and intake are too close together, as is the case in my 1988 Sunbeam, shortcutting will result.

Shortcutting

In the photo notice the paper strips, showing that the wind is blowing strong from the exhaust vent on the right.

See how close the intake and exhaust ports are?

Notice how the left paper strip is starting to curve toward the middle?

The red arrows show the path that some exhaust takes, right back into the intake.

This little Sunbeam is in a place I call the purifier's worst nightmare, the living area of my dusty country home.

There is a 16 foot ceiling, with a high window facing the morning sun.

As the sun rises, a strong shaft of light cuts through this room, perfect for visual evaluation. It is very easy to see the behavior of particles up to ten feet from intake and exhaust ports.

The old Sunbeam can easily be seen to chase its own tail, shortcutting and failing to pull particles from a distance.

Downdraft exhaust, which stirs up dust, can also hinder a unit's ability to purify a larger area.

Air Changes Per Hour

Most air purification experts and allergists recommend at least 2 room air changes per hour (ACH).

I recommend much higher power-to-room-size ratios, with burst mode response to real world peaks in air pollutant levels followed by quiet interludes.

If your machine has power for my recommended six ACH, it will take somewhat over 10 minutes (1/6 hour) to purify the air on high, assuming zero reintroduction.

Nooks and crannies, closets, and dusty spaces under beds resist cleaning. Lower powered air purifying units will need to run on noisy high speeds most of the time.

Six ACH means a 12 by 14 bedroom with an 8 foot ceiling (1344 cubic feet) needs 224 cubic feet per minute from the air cleaner.

A majority of models on the market are weaker than this.

Using CADR as a rough approximation of cubic foot per minute output, of approximately 225 models room size certified by AHAM, only 43 purifiers were rated at or above 225 cfm.

Size ratings in advertising, like CADRs, should be used only to compare among similar powered and priced units, and deflated significantly for all other purposes.