Fuel fleas are microscopic hot particles of new or spent nuclear fuel. While small, they tend to be intensely radioactive. The fuel particles, the size about 10 micrometers, are a strong source of beta and gamma radiation and a weaker source of alpha radiation. The disparity between alpha and beta radiation (alpha activity is typically 100–1000 times weaker than beta, so the particle loses much more negatively charged particles than positively charged ones) leads to buildup of positive electrostatic charge on the particle, causing the particle to "jump" from surface to surface and easily become airborne.