In its latest round of national emission standards for hazardous air pollutants (NESHAP), Environmental Protection Agency proposes lower limits for mercury and non-mercury metals, plus hydrogen and chlorine gases from clay brick tunnel (continuous curing) kilns, coupled with target startup and shutdown periods. EPA also envisions work practice standards for more specialized periodic kilns, which are heated up to 2,000°F, then left to cool for several days. Tunnel kilns are typically fired by natural gas or sawdust; periodic kilns, coal.