The main hazard of ventless gas fireplaces is that there is no flue or chimney to expel gases to the outside, which may result in measurable traces of chemical residue such as carbon monoxide remaining in the exhaust and affecting indoor air quality. Exposure to ventless gas fireplace chemical pollutants in the air may be hazardous to the health of those exposed to them, with symptoms including congestion, headaches and asthma attacks.
Other traces of airborne chemicals a ventless gas fireplace emits may also create a slight odor, which many people only notice for the first few minutes after the fireplace turns on, while others compare to the smell of a kerosene heater. These chemicals are a hazard for the respiratory system, especially for those who suffer from sensitivities such as allergies and asthma.
All ventless fireplaces sold in the United States must meet general requirements for combustible heating appliances as part of National Fire Protection Association standards. These standards require that all ventless fireplaces have factory-installed oxygen detection safety devices and carbon monoxide monitors that automatically shut off the fireplace if the immediate environment does not meet certain indoor air quality levels. As suggested by the U.S. Environmental Protection Agency, ventless fireplaces shut off when the level of carbon monoxide rises above 25 parts per million or the oxygen level falls below 18 percent.