Alpha and beta radiation are particle radiation types. An alpha particle consists of two protons and two neutrons. Alpha decay typically occurs in heavy nuclides such as natural uranium or thorium. Spent nuclear fuel contains elements that are heavier than uranium, called transuranic elements, which emit alpha radiation. Beta particles may be electrons or positrons. Electrons have a negative charge and positrons a positive charge. Examples of elements that emit beta radiation include cesium-137 and strontium-90, which are created when uranium undergoes fission. Alpha particles are heavier than beta particles. An alpha particle is not able to penetrate human skin or a piece of paper. Alpha radiation is only dangerous if radioactive substances emitting alpha radiation are ingested by breathing air or consuming food, for example. Unlike alpha particles, beta particles are able to penetrate skin. Plastic offers protection against beta radiation; clothes may also be able to stop beta radiation. Substances that emit beta radiation are able to cause damage through skin contact and when ingested. The daughter nucleus created by alpha or beta decay is often in an excited state and emits gamma radiation when the state disintegrates. Gamma radiation is not particle radiation, but consists of electromagnetic waves. Gamma radiation generally has a high penetrating capacity, and a thick layer of concrete, steel, or lead, depending on the energy level of the radiation, is required to protect the environment from gamma radiation. If the energy level of the gamma radiation is low, a layer of lead only one millimeter thick is adequate.