Hygrometer
The range
30–50% relative humidity, for spaces people occupy. That is where ASHRAE's comfort work and the EPA and WHO damp-building guidance converge, and it is the band the app's own comfort scale is built on.
| Band | Range | What happens here |
|---|---|---|
| Dry | Under 30% | Static, chapped skin and irritated airways become common. |
| Comfortable | 30–50% | The healthy range. Comfortable for people, hostile to dust mites and mould. |
| Moderate | 50–60% | Still fine, heading up. Evaporation slows as this climbs. |
| Humid | 60–70% | Sustained readings here let mould and dust mites take hold. |
| Damp | Over 70% | Close to saturation. The air gives up water onto any cool surface. |
Why both ends have a limit
The low end is about you. Dry air pulls water out of whatever it touches, including skin, lips, eyes and the mucous membranes lining the airways. The effects are gradual rather than threshold-like, which is why 30% is a convention rather than a cliff — it is roughly where most people start noticing. Timber, glued furniture and instruments also shrink and crack in air this dry.
The high end is about biology. Dust mites need around 50% ambient humidity to draw water from the air and cannot survive sustained conditions below it. Mould needs roughly 80% at a surface, which a room averaging 60% will readily produce in cold corners and against glass.
So the upper limit is not about discomfort at all. It is about what else starts living there.
The temperature problem
The 30–50% range is stated for ordinary room temperatures, and it does not travel well.
The same percentage is a very different amount of water at different temperatures, so a rule written in percent quietly assumes one. At 68°F, 50% is 8.6 g/m³. At 86°F it is 15.1 g/m³ — nearly twice the water, at the same "50%".
Which is why comfort in warm conditions is better described by the dew point: under 55°F feels pleasant, over 65°F feels muggy, at whatever percentage. Above about 75°F the percentage stops being a useful comfort measure at all.
Where the percentage is still the right measure
Not everything should be converted to dew point. Relative humidity is the correct measure whenever the question is about a rate or a surface:
- Mould and dust mites, whose thresholds are properly defined at surfaces.
- How quickly things dry.
- Static electricity, which depends on the water film on surfaces.
- Timber and instrument stability, which track the equilibrium moisture content of the material.
All of those depend on how close the air is to saturation, which is exactly what the percentage measures.
What this site can tell you
Outdoor conditions — the dew point, the water content, the comfort band of the air outside. That is genuinely useful for whether the evening will feel close, whether anything will dry, and whether ventilating will help or hurt.
For indoors, the app keeps a list of rooms. With a HomeKit humidity sensor it reads the room directly; without one it estimates the likely range for a room of that kind from the outdoor conditions and how the room is used. No phone has a humidity sensor of its own, so an estimate is what you get until there is an instrument in the room.