Lead is released from lead pipes in both soluble and particulate forms. Soluble lead concentrations develop over a 24 hour period through a process of diffusion from the pipe wall . Ultimately soluble lead concentrations are limited by equilibrium with the lead compounds that make up the corrosion layer that protects the metal from the water . Many factors influence soluble lead concentrations (e.g. chemical composition of water, temperature) and this contributes to the observed variation in lead concentrations within a property . Particulate lead is an additional cause of variation. There are at least two types of particulate lead ). One type is associated with iron particles that adsorb lead and the other is particles that flake off from the corrosion layer. By its very nature particulate lead is erratic and accounts for the occasional detection of a sample with a very high lead concentration.