We air our homes to refresh the air, but we often assume that cool air is dry air. It is not: what matters is the amount of water vapour in the air, which the dew point measures, not the temperature.
Why this indicator matters
Outdoor air at 5 °C and 90 % humidity contains less water than indoor air at 20 °C and 50 %. Once it comes in and warms up, it becomes dry: relative humidity falls, because warmer air can hold far more vapour. Conversely, a muggy summer evening at 24 °C can bring a dew point of 17 °C: by opening the windows, you let water in.
That water has a cost. A home that stays above 60 % humidity for several days in a row becomes a good breeding ground for dust mites, whose droppings are the most common household allergen, and then for mould on cold walls. Airing at the right moment is the cheapest way to protect yourself.
How to read it
The box gives a slot of no more than three hours, taken between 6:00 and 22:00 over the next twenty-four hours: the one where the outdoor dew point is lowest. It compares the outside air, warmed to 20 °C, with a reference indoor space at 20 °C and 50 % humidity, whose dew point is 9.3 °C.
When the dew point in the slot is below 11 °C, opening the windows dries the house or, just above 9.3 °C, changes almost nothing. Above 11 °C, the sentence tells you to air for only five minutes. An extra sentence about dust mites appears when the air stays damp for at least three days in a row.
The thresholds used on this site
Every sentence in the “Today” box comes from a numerical rule. Here are the rules for this indicator, exactly as they are applied in the code.
- Outdoor dew point below 11 °C: the outside air dries the indoor air; above it, it adds moisture.
- Reference indoor conditions: 20 °C and 50 % relative humidity.
- Slots searched between 6:00 and 22:00, no more than 3 hours long, allowing a 1 °C margin above the minimum.
- Dust mites: estimated indoor humidity above 60 % for at least 3 days in a row, over 5 days of forecast.
A worked example: Toronto
In Toronto, with the forecasts of 2026-11-02 08:13: Best time to air the house: around 8:00. The outdoor dew point drops to 6.1 °C; once warmed to 20 °C, that air holds only 40 % humidity, against 50 % in a typical home: opening the windows dries the house.
Its limits
The reference indoor space is an average home: a kitchen after cooking, a bathroom or a cellar have different humidity, and a hygrometer costing a few pounds is worth more than any model for your own room. The calculation ignores wind direction and which way the windows face. It describes the outdoor air at the town's location, not in a narrow, sheltered street.
Frequently asked questions
Is it always a good idea to air the house when it is cold outside?
Almost always: in winter, outdoor air, even damp air, contains little water and dries the indoor air once it has warmed up. The only exception is the mild fog of the in-between seasons, whose dew point can exceed 11 °C.
How long should I air the house?
Five to ten minutes with the windows wide open to create a through-draught is enough to renew the air in a home. Leaving a window tilted open for hours cools the walls without really renewing the air.
Why 60 % humidity, and not 50?
Between 40 and 60 % the air is comfortable and not very welcoming to dust mites. Above 60 % they multiply; below 30 %, your airways suffer. The 60 % mark is the one used to trigger the warning.