What is formaldehyde? Formaldehyde is a toxic flammable gas commonly used in the manufacture of furniture, toys, decorative building materials and other products. Long-term exposure to formaldehyde can cause adverse effects on the human body and is therefore listed as a carcinogen by the World Health Organization. What are the effects of long-term inhalation of formaldehyde on pregnant women? Long-term exposure of pregnant women to formaldehyde will directly affect the fetus, causing fetal malformations during development and even causing chromosomal abnormalities.