Air Quality

There are many different pollutants that influence air quality: gasses like: carbon-monoxide (CO), carbon-dioxide (CO₂), or volatile organic compounds (VOC); or particles with a diameter of less than 2.5 µm (PM_2.5) or 10 µm (PM₁₀).

An air quality sensor measures one or more pollutants. Each pollutant is reported by level (usually in ppb), or by density (usually in µg/m³). These quantities are obviously related, but the conversion depends on the pollutant as well as on the air temperature.

Different pollutants have different threshold values for air quality levels. Worse: different authorities use different thresholds and even different levels. Consequently, different models air quality sensor might report different air quality levels, even when measuring the same pollutant.

In HomeKit, AirQuality summarises the quality of the air into one of five levels: Excellent, Good, Fair, Inferior, Poor. Depending on the pollutant, HomeKit includes a characteristic for the level or for the density. The mapping of the pollutant level or density to the air quality level is left to the accessory.

Eve Room

The Eve Room measures Volatile organic compounds (VOC).

The first generation Eve Room reports the level in ppb. It uses the custom VOCLevel (10B), with values from 5 to 5000, in steps of 5. This corresponds to history tag 04.

The second and third generation report the density in µg/m³. They use VOCDensity. which corresponds to history tag 22. The Eve app still displays the level in ppb instead of the density in µg/m³. The level is computed by multiplying the density by 4.57, assuming a temperature of 20°C.

AirQuality corresponds to history tag 28. It is derived from VOC Level or VOC density:

Value	Description	Stars	VOC level ppb	VOC density µg/m³
1	Excellent	5	1 - 119	5 - 545
2	Good	4	120 - 199	550 - 910
3	Fair	3	200 - 359	915 - 1640
4	Inferior	2	360 - 719	1645 - 3285
5	Poor	1	720 -	3290

IKEA Starkvind

The IKEA Starkvind air purifier measures PM_2.5 density in µg/m³.