Every asthmatic should be aware that both humidity and cold air are two very common asthma triggers. So why is this? What can you do about it?

It's been common wisdom for years that the combination of humidity and cold air helps with croup, or swelling of the voice box and trachea.   Put a croupy kid in the hot and steamy bathroom and the swelling gets better.

Another method that often works for croup is taking the child outside in the cold winter air.  This is why many times when a parent decides to take the child to the hospital, the child is fine by the time they arrive in the emergency room.

This is true for croup, so many doctors of old believed it must also be true for asthma. Yet it was a fallacy, and now -- thankfully -- most doctors are aware of this fallacy. In fact, now doctors are aware that both cold air and humidity can actually trigger an asthma attack.

When I was little boy way back in the 1970s, my pediatrician recommended my parents have me sit in the hot steamy bathroom when I was having trouble breathing. It was also recommended I have a humidifier in my room.

Both of these made my asthma worse, not better. Yet I was a kid, so how was I to tell my parents that?  My doctor and parents thought they were doing something good, yet their wisdom was flawed.

I wrote a post before how low and high humidity can trigger asthma. Studies show that a humidity of 50 percent or greater may lead to a greater incidence of asthma trouble.

Two common theories for this are:

• Humid air is heavier and harder to breathe
• Humid air harbors fungus, molds and dust mites that trigger asthma

Humid air is most often a problem in the summer months, especially in August and September.

(On a side note here, when I was a kid there was also a fear that air conditioners were bad for asthma.  That was a fallacy that led to many uncomfortable August car rides.)

As I wrote before, I also remember having asthma trouble when my brothers and I would go sledding. I'd usually have to quit early and arduously walk home with my asthma symptoms raging.

Now we have research that shows air that is too dry can also trigger asthma. Air tends to be drier in the winter months. The reason is that the colder the air, the less water it can hold.

When you inhale cold, dry air, it can dry the mucus membranes lining your lungs, which are your body's natural defense mechanisms against viruses and bacteria. So this can lead to increased infections, too. And we know viral infections are the most common asthma trigger.

Dry mucus membranes can also aggravate allergy symptoms. And considering 75 percent of asthmatics have allergies, this is important.

Now that you understand dry air can trigger asthma, consider the following:

1. Exercise can trigger asthma: As I write in more detail here, rapidly breathing in air dries inspired air, which ultimately dries the airway, which then releases histamine that can increase inflammation of the air passages in your lungs. This then leads to bronchospasm. The fact runners tend to breathe through their mouths only exacerbates this problem because the nose is a better humidifier than the mouth.