Four-Hour Block Average Law and Legal Definition
Four-hour block average or 4-hour block average means “the average of all hourly emission concentrations when the affected facility is operating and combusting municipal solid waste measured over 4-hour periods of time from 12:00 midnight to 4 a.m., 4 a.m. to 8 a.m., 8 a.m. to 12:00 noon, 12:00 noon to 4 p.m., 4 p.m. to 8 p.m., and 8 p.m. to 12:00 midnight.” [40 CFR 60.51b; Title 40 -Protection of Environment; Chapter I -Environmental Protection Agency; Subchapter C - Air Programs; Part 60-Standards of Performance for New Stationary Sources]
