What is the primary role of formaldehyde in beauty products?

Formaldehyde's primary role in beauty products, particularly in the context of hair treatments such as keratin treatments, is to fix and preserve the structure of the hair. It acts by creating cross-links between the protein molecules in the hair, which helps to maintain the straightened or styled look achieved during the treatment. This fixing function provides longevity to the hairstyle, preventing it from reverting to its natural form quickly.

In contrast, the other choices focus on different attributes of hair care products. While many products claim to moisturize or strengthen hair fibers, these functions are primarily managed by ingredients that enhance hydration and protein content, rather than through the fixing capabilities of formaldehyde. Softening hair strands is typically associated with conditioning treatments that use emollients and moisturizers, which is not the role of formaldehyde. Thus, understanding the specific function of formaldehyde in the context of hair treatments clarifies why its primary purpose is to fix and preserve hair structure.