Every building material contains traces of natural radioactive substances, especially uranium, thorium, and their decay products, and potassium. Solidified and effusive rocks such as granite, tuff, and pumice have higher levels of radioactivity. In contrast, sand, gravel, limestone, and natural gypsum (calcium sulfate dihydrate) have low levels of radioactivity. The European Union's Activity Concentration Index (ACI), developed in 1999, can be used to assess radiation exposure from building materials. It replaces the Leningrad summation formula, which was used in 1971 in Leningrad (St. Petersburg) to determine how much radiation exposure from building materials is permissible for humans. The ACI is calculated from the sum of the weighted activities of potassium, radium, and thorium. The weighting takes into account the relative harmfulness to humans. According to official recommendations, building materials with a European ACI value greater than "1" should not be used in large quantities.