Can you scuba dive with a cold or a blocked nose?

Your middle ears and sinuses are air spaces connected to your nose and throat. To equalise, air has to pass through narrow openings — the Eustachian tubes and the sinus passages. Swelling and mucus from a cold can narrow or block them.

DAN also notes that cold water itself triggers extra mucus production, and that when you're recovering from a cold, moving gas in and out of the sinuses may be significantly harder than usual.

If a passage is only partly blocked it can act like a one-way valve: air gets in on descent but can't escape as it expands on ascent. The result is pain and pressure in the ear or face while you come up — and you can't simply stay down. As DAN puts it, you can always abort a descent; aborting an ascent is a lot more problematic.

Decongestant questions are among the most common calls to DAN's medical information line. Decongestants are not suitable for everyone and are no guarantee: a medicine that wears off during the dive can leave you with a blocked passage on the way up. Talk to a doctor before diving on any medication, and don't use one to push through a cold.

Stop descending, ascend a little, and try gently again. If it still won't clear, end the dive. DAN calls repeatedly bobbing up and down to force it 'counterproductive' — forcing air against swollen tissue only locks it shut.