Copper oxide is formed when copper is exposed to oxygen and oxidizes. There are two types of copper oxide: copper (I) oxide and copper (II) oxide. Copper (I) oxide is referred to as cuprous oxide, while copper (II) oxide is known as cupric oxide. Both are used as pigments and can be used as semiconductors. Even though they have the came components, they each have slightly different characteristics.

Cuprous oxide has the chemical formula Cu₂O. It is red in color and does not dissolve in water or any organic solvents. In nature, this copper oxide is commonly found in rocks as the mineral cuprite. However, the production of cuprous oxide in the natural world takes a while to happen. Therefore, it can be manufactured artificially at high temperatures or under high oxygen pressure.

Cuprous oxide can also be used as a pigment, as a fungicide, and as an anti-fouling agent for marine paint. It is also used as a catalyst. However, exposure to this type of copper oxide can be dangerous. If inhaled, it can cause shortness of breath, coughing as well as ulceration and perforation of the respiratory tract. If ingested, it can cause irritation of the gastrointestinal tract, vomiting, pain, and diarrhea.

Cupric oxide (CuO), on the other hand, is a black material that melts above 2192°F (1200°C). This copper oxide is used as a pigment in clay glazes. Several colors, including red, blue, and green, can be derived from it. It is also used to produce cuprammonium hydroxide, a material that is used to make rayon. Some also give this form of copper oxide to animals as a copper supplement.

Exposure to this type of copper oxide can also be dangerous. If inhaled, cupric oxide can cause irritation to the lungs. Cupric oxide, as well as cuprous oxide, can also cause metal fume fever. Symptoms of the fever are flulike and temporary. Cupric oxide can also cause skin discoloration and can cause vision problems. Like cuprous oxide, it is also toxic when ingested, causing symptoms like vomiting and pain.