Formaldehyde
Danger! According to the harmonised classification and labelling (ATP06) approved by the European Union, this substance is toxic if swallowed, is toxic in contact with skin, causes severe skin burns and eye damage, is toxic if inhaled, may cause cancer, is suspected of causing genetic defects and may cause an allergic skin reaction.
Additionally, the classification provided by companies to ECHA in REACH registrations identifies that this substance is fatal if inhaled and causes serious eye damage.
Source and more information: https://echa.europa.eu/substance-information/-/substanceinfo/100.000.002