It is bad to let your hair air dry as air drying your hair can cause trauma to the inner lipid layer and make your hair strands weaker as a result.
After washing your hair it's best to use a hair dryer to dry the hair.
But if you don't have a hair dryer then you will be fine with letting your hair dry other than possible damage to the inner lipid layer which can lead to weaker strands.
I've let my hair air dry before without any issues but some people do have issues with weaker hair strands.
I most often dry my hair with a hair dryer after washing but sometimes I forget or am too tired and let it dry naturally.
Keeping your hair wet for long periods of time (for example, overnight or air drying on a regular basis) can cause hygral fatigue, which is damage to the hair from excessive swelling.
It is also best to allow your hair to dry completely before going to sleep because sleeping with wet hair is not healthy for your hair strand or scalp.
Continuously sleeping with wet hair can lead to increased breakage, frizz, split ends, dryness, dandruff, and various scalp health issues.
Drying your hair after washing your hair may seem like a simple task.
However, drying your hair after washing it is a crucial step in maintaining the health and appearance of your hair.
After showering or washing your hair, your wet hair is in a fragile condition.
As a result your hair then becomes more vulnerable to breakages and other damages.