Experts differ on how safe ventless natural gas and propane heaters are, but vented gas heaters are at least as safe as their ventless counterparts if they are working correctly. Blocked ventilation, however, can make vented heaters dangerous.
If combusted fully, natural gas and propane produce only trace amounts of carbon monoxide and other dangerous chemicals. Most newer ventless models can detect some dangerous operating conditions. Vented heaters release any dangerous chemicals produced outside the structure, making them completely safe with regard to air quality, but failure in the ventilation system can leak dangerous chemicals inside the home. Regardless of whether a vented or ventless model is used, a carbon monoxide detector can provide peace of mind.