Mercury (element) elevated over outdoor concentrations, at a range of 0.0065 – 0.523 μg/m. The average was 0.069 μg/m. Mercury also enters into the environment through the improper disposal (e.g., land filling, incineration) of certain products. Products containing mercury include: auto parts, batteries, fluorescent bulbs, medical products, thermometers, and thermostats. Due to health concerns (see below), toxics use reduction efforts are cutting back or eliminating mercury in such products. For example, the amount of mercury sold in thermostats in the United States decreased from 14.5 tons in 2004 to 3.9 tons in 2007. Most thermometers now use pigmented alcohol instead of mercury,
Period 4 element toxic to humans in large quantities, and <nowiki>bromine</nowiki>, a very toxic liquid. Many elements are essential to humans' survival, such as calcium being what forms bones. (*) Exception to the Madelung rule Potassium (K) is an alkali metal, placed under sodium and over rubidium, and is the first element of period 4. It is one of the most reactive elements in the periodic table, therefore usually only found in compounds. It tends to oxidize in air very rapidly, thus accounting for its rapid reaction with oxygen when freshly exposed to air. When freshly exposed, it is rather silvery, but it