The U.S. Food and Drug Administration (FDA) announced last week that it cannot ensure the safety and quality of washable hair chalks sold through the internet because the agency has not received a notification that the products comply with health and safety standards.

"The products may contain ingredients such as dyes or colorants, chemicals and preservatives, which are not allowed and may cause allergies and adverse reaction on the scalp, head, eyes and skin," said the FDA in an advisory note.

Hair chalking is a way to add temporary color to your hair without permanently dying it. The trend is especially popular among teens and kids because the colors are vibrant — including blues, purples, reds, and yellows to name a few — and the process of putting the color in is relatively easy. There's no need to go to a hair dresser to get it done. All you need is the chalk itself and water really.

"Hair chalking is the recent craze among our teenagers, which involves rubbing a piece of colored pastel chalk onto certain strands of hair to change their hair color. For dark-haired persons, hair strands to be colored must be moistened first with water then twisted on sticks as the chalk is being applied and lock in the color by running through a hair straightener iron," FDA director general Kenneth Hartigan-Go wrote.

Hair chalk is not expensive. The prices start at about $14 at stores like Urban Outfitters and go up from there.

Though the products appear harmless, the FDA urges the public not to use products, including cosmetics, that have not been approved for public use by the agency.