A humidifier is not recommended for people with asthma.

More info: Though a humidifier has been recommended for people with respiratory complaints, they are not recommended for people with asthma.  Dust mites, which are a common trigger for asthmatics, thrive in humid air so your humidifier may make your situation worse.  In fact, rather than recommending humidifiers for asthma, the opposite is true.  Using a dehumidifier can help to keep your surrounding dry and will help to lower the dust mite population.

Management of Asthma. The Ohio State University Medical Center

What Are the Advantages of Using a Humidifier?

Humidifiers are often recommended in the relief of certain respiratory conditions.  A low moisture level in the home can irritate some causing itchy, watery eyes, irritated nasal passages, and dry skin so adding a bit of moisture to the air can often relieve these minor dry air irritants.

What Are the Disadvantages of Using a Humidifier?

Humidifiers are devices that add more moisture to the air. They can often make an entire room or home more humid, and many people use humidifiers when they are having respiratory issues. Unfortunately, a humidifier can also cause some problems in the home, with the extra moisture leading to mold growth and an excess of dust mites. Those who have hard water in their homes may encounter issues with dehydrated calcium being released into the air in their homes due to the humidifier. Inhaling too much moisture indoors over a prolonged period of time can also lead to inflammation in the lungs.