In this case, both allotropes are gases. Ordinary oxygen, as in the air we breathe, consists of molecules each of which contain two oxygen atoms. The other form of oxygen is ozone, which has three oxygen atoms in each molecule. Ozone can be formed from oxygen by passage of an electric discharge. The peculiar smell around model train transformers is due to ozone.
Ozone is extremely reactive chemically and is very poisonous.
As little as one part of ozone in a million parts of air causes a choking sensation and difficulty breathing for many people. This is a part of the Earth's atmosphere, about 30-80 km in altitude, where small concentrations of ozone are present. The ozone is formed from oxygen by high energy ultraviolet radiation from the Sun. The ozone in turn is also a ultraviolet-radiation absorber. Although there is rather little ozone present, its importance for life on Earth is immense because of its ability to absorb this harmful ultraviolet radiation which would otherwise reach the Earth's surface.