What effect does humidity have on hair?

Humidity has a significant impact on the structure and appearance of hair due to the moisture content in the air. When humidity levels rise, hair absorbs water from the environment, leading to swelling of the cuticle layers of the hair shaft. This absorption causes the hair to expand and can disrupt the smoothness of the hair strands, making them more frizzy and unruly. The elevated moisture can also create a lift in the hair, which contributes to a frizzy appearance.

On the other hand, the other options present contrasting effects that do not align with the scientific understanding of hair and humidity. For example, hair does not become smoother and shinier with increased humidity; rather, it often becomes more textured and prone to frizz. Similarly, humidity does indeed affect hair, so stating it has no effect is inaccurate. Finally, while hair may seem to behave differently in changing conditions, humidity does not strengthen hair strands; in fact, overly humid conditions can sometimes lead to damage as hair becomes more susceptible to breakage when it's frizzy and tangled. Thus, the swelling and frizzing caused by humidity is the most accurate description of its effect on hair.