Before applying a coating to a fresh concrete slab, it should be allowed to fully cure for 30–60 days. If the concrete has previously been sealed (water beads up on the surface), do not apply epoxy. Before applying a new epoxy coating, any cracks or concrete spalls should be filled and stabilized. Cracks can be filled with concrete repair materials or other types of joint fillers. Concrete that is still wet may emit toxic fumes when heated or exposed to heat. These chemicals are called volatile organic compounds (VOCs). When heated, some VOCs may release into the atmosphere. Heated concrete also releases ammonia, which is harmful if it enters the air you breathe. Ammonia gas is brown in color and smells like soap. It is very dangerous if it mixes with water to form ammonium hydroxide. Ammonia is released when fresh concrete surfaces are screed, leveled off with a roller blade or smoothed by hand. This process removes some of the roughness caused by the sand and stone used to make the concrete mix. The smoother the concrete, the faster it will dry and the better it will resist cracking and chipping. The best way to protect freshly mixed concrete from ammonia fumes is to allow it to cure completely before filling cracks or potholes with asphalt patching material or other filler products. This will prevent further emissions until the concrete has time to cure completely.