Although SIC codes are being replaced by NAICS codes in building emission inventories at EPA, SIC codes are still used in emissions processing. As of SMOKE v4.0, the SIC codes may be up to 20 characters in length, but for SIC codes greater than 4-digits, the hierarchial system is not used. For SICs of size 4-digits, a 2-level hierarchial system is recognized by SMOKE for application of growth, control, and chemical speciation factors. The two code levels are illustrated below.