Q:

How cold can an asthmatic's drinking water be?

For an asthmatic, drinking cold water can apparently trigger (or worsen) an asthma attack. (See here for example.)
How cold is cold water for this purpose?
I'm trying to find some threshold, between room temperature and ice cold.

A:

Drinking cold water having asthma can cause bronchial spasm, a sudden constriction of the muscles in the walls of the bronchioles (tightness in the chest), causing difficulty in breathing. This can be also triggered by Exercise-induced asthma (EIA).

The neurotransmitter acetylcholine is known to decrease sympathetic response by slowing the heart rate and constricting the smooth muscle tissue. Ongoing research and successful clinical trials have shown that agents such as diphenhydramine, atropine and Ipratropium bromide (act as receptor antagonists of muscarinic acetylcholine receptors) are effective for treating asthma and COPD-related symptoms.wiki

Studies from 2000 has been shown that both exercise and drinking ice water can induce airways obstruction, however there is a limited interpretation of results, because of the difficulty of separating the temperature and humidity of the airways during experiments (where airway microcirculation could be an important factor).
So how cold the water should be? I don't think it's possible to answer and it really depends on the individual organism and many other factors. If more studies would follow, it would give you only the averages, nothing more.
The following study used ice water (0-4°C) and warm water 37°C:

Effect of ice water ingestion on asthmatic children after exercise challenge. (2000)

Thirty Chinese asthmatic children underwent exercise challenge by ergocyclometer for 6 minutes and then were further challenged by immediate ingestion of ice water (200 ml, 0-4 degrees C), warm water (200 ml, 37 degrees C) or no ingestion on three different days in one week. Each patient completed the three different water ingestion tests after exercise challenge.
Exercise-induced asthma (EIA) developed in about two thirds of the 30 patients, regardless of whether ice water, warm water or nothing at all was ingested after exercise challenge. There was no statistically significant difference in spirometric data among the 3 different water tests at various time points.
A statistically significant difference was found between ice water and warm water tests for FEV1 and PEF (p = 0.0293 and p = 0.0308 respectively).
Those who ingested warm water after exercise had a better bronchodilator response than those who ingested ice water.

If they had ice in it - I don't know. However based on the above you should drink warm water just to be safer.

