Breathing the fumes of antifreeze can cause, according to the Centers for Disease Control and Prevention, "eye and respiratory tract irritation but is unlikely to cause systemic toxicity." The harmful chemical within antifreeze that causes these side effects is called ethylene glycol.
The active chemical in antifreeze, ethylene glycol, poses additional hazards in addition to harmful fumes. The CDC notes that beyond breathing the fumes, exposure to the eye is also a significant risk. Such exposure would result in localized discomfort, but would not pose a risk of systemic toxicity. Ethylene glycol is poorly absorbed through the skin, which means it would be difficult for the chemical to cause systemic toxicity.
Ethylene glycol reacts with oxidants and acids, particularly strong oxidants and acids. It is also a highly flammable chemical. Because ethylene glycol vapors are heavier than air, they can collect in poorly-ventilated areas, particularly those that are below ground level. When using ethylene glycol, one must be cautious to do so in a well ventilated area as to not let the fumes collect. In the case of skin exposure, ethylene glycol will generally only cause mild irritation. The fumes of the chemical are far more harmful than physical contact with the substance. In the case of skin exposure, soap and water should be used to prevent irritation.