This is a bit of a tricky question. Some acrylics contain formaldehyde while others do not. Even some of the ones that do not contain the chemical may be treated with it. This is particularly common in carpets. As a person with severe allergies, I would advise testing the material before using the yarn to knit any items of clothing. If the product is important enough to you, you can check with the manufacturer to ensure that the product does not contain formaldehyde. Frankly, I'd still do an allergy test in case some similarly toxic chemical or irritant may have been used in the manufacturing process.