The air is too dry or too damp, and a hygrometer beside the bed will say which
The air in this bedroom is probably too dry or too damp, and a hygrometer read beside the bed over several nights will show which one it is.
In short: Read a hygrometer beside the bed over several nights: under 30% is too dry, and over 50% is damper than the range the EPA advises.
What to check
| Check | Specification |
|---|---|
| What to measure | Relative humidity at the bed, read at bedtime, in the small hours and on waking, with the room temperature noted next to each reading. |
| The instrument | A digital hygrometer, which the EPA describes as a small, inexpensive ($10-$50) instrument available at many hardware stores (EPA). Check it against a known point before relying on it. Damp table salt sealed in a bag with the meter holds the air at about 75% relative humidity, a fixed point from the US National Bureau of Standards. |
| The range that matters | If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity (EPA). On the dry side, a review found that RH about 40% is better for the eyes and upper airways than levels below 30% (Wolkoff and Kjærgaard). |
| Where to put it | On the bedside table at about the height of your head on the pillow. Keep it off the windowsill and away from the radiator, the vent and the outside wall. |
| What to change first | If the air is dry, turn the night heat down or move the bed away from the vent first. If you add moisture, do not humidify to indoor relative humidity levels exceeding 50 percent (EPA). If it is damp, find the water first, then repair leaks, increase ventilation (if outside air is cold and dry), or dehumidify (if outdoor air is warm and humid) (Colorado Department of Education, citing EPA). |
Why the answers point here
Two different complaints lead to this page, and they sit at opposite ends of the same reading. A dry throat, nose or lips on waking, worse in winter and in a room warmed by vents or a radiator, points to the dry end. Warming air without adding water lowers its relative humidity. A smell you cannot place, in a room where nothing new has arrived, points to the damp end. A ground-floor or basement room, a shut window and a problem that lasts all year all push in that same direction. A closed door and window also keep the moisture from a night of breathing inside the room, which matters more on the damp side than the dry one. Both signs can mislead, since a dry throat has other causes and so does a smell, which is why the reading decides and the symptom only starts the search. The guide to ideal bedroom humidity covers the range in more depth.
What changes the reading
Temperature moves the number. The same air reads drier next to a radiator and damper beside a cold outside wall, so the meter belongs at the bed and not on the sill. The humidity close to surfaces can also run above the room's figure: Lawrence Berkeley National Laboratory notes that the relative humidity where mites live can be higher than the average indoor humidity (LBNL). That matters for the mattress, since a review of mite water balance reports that active mites do not survive longer than 6-11 days at RHs < or = 50% (Arlian, 1992). The dust mite guide explains what that means for bedding.
The meter brings its own doubt. A good consumer hygrometer is accurate to roughly ±2 to ±5 percentage points of relative humidity, and the salt test verifies a single point, at 75% RH (Hygrometer.net). If a reading falls near 30% or 50%, treat it as a range. The meter you rely on may be a few points off at that end of the scale.
What to check on your own bed
Start with the glass, and on a cold morning look at the window and at the wall behind the headboard for water. The EPA advises that if water condenses on windows, walls, or pictures, relocate the humidifier, lower its humidistat setting, or reduce its use. The same sign in a room with no humidifier points at damp. Pull the bed away from the wall and smell the gap and the underside of the mattress, since a damp smell often starts where the air does not move. If the reading is dry, watch whether it falls when the heating comes on at night, and lower the setting before adding any water. If you find the room damp in winter, the guide to ventilating a bedroom in winter covers airing it without losing the heat.