Formaldehyde is a colorless gas with a pungent, unpleasant odor. It is found in many household products such as glues, adhesives, paper product coatings, and in some insulation materials. It is used as a bacterial disinfectant and embalming fluid, and it's a common treatment for warts since it dries out the skin.
Formaldehyde is also a preservative often found in nail polish. It helps harden the lacquer and prevent it from chipping. Unfortunately, it is highly flammable, toxic and is proven to be carcinogenic in animals (with studies linking it to human cancer too). Breathing in formaldehyde can be detrimental to your lungs, eyes, throat and nose when inhaled. It can also lead to headaches, fatigue and dizziness.
Since there are many formaldehyde-free nail varnishes on the market, it's important to be aware of labels and to choose your products wisely. And I would stay far away from hair-straightening treatments that use formaldehyde. After all, beauty products are intended to enhance your body, not harm it .