Dew point

What it is

The temperature at which the water vapor in the air would condense into a dew drop. If it’s too high, the hot, wet air can set off a reflex that tightens airways. If it’s too low the dry air causes the lining of the airways to dry out. In between is comfortable.

How it affects breathing

• Evidence. Two mechanisms, both shown in lab studies. On the dry side, below a dew point of about 11 °C (52 °F), what people call cold-air asthma is drying, not cold, and it needs hard breathing to start: hard exercise in dry air tightens the airways of most people with asthma within minutes. On the humid side, above about 18 °C (64 °F), a separate reflex that a drug in some inhalers blocked in the lab; it also needs hard breathing.
• How likely. The dry side is a winter problem and the humid side a July-and-August one. Both are among the most common asthma triggers there are, and both are the easiest to get out of.
• What helps. On the dry side, breathing through the nose nearly cancels it, a scarf or mask over the mouth warms and wets the air, and a rescue inhaler taken 15 minutes before exercise prevents it in most people. On the humid side, air conditioning removes the trigger.

How Breathing Index measures it

Your Breathing Index just takes into account the hour at which the dew point is measured; there isn’t a cumulative effect to take into account.

Where the number comes from

The weather forecast’s hourly dew point. Always a model number, because no air-quality monitor reports it.