During the towers' construction, a ban was imposed on the use of fireproofing materials containing more than 1% asbestos. The towers were under construction at the time, with floors 38-40 of 1 WTC completed. The remaining floors and buildings were constructed using fiberglass and mineral wool, as well as concrete and gypsum plasterboard. Asbestos removal began in 1986, and over the next 15 years, 18 of the 38-40 floors were removed, replacing them with new fireproofing. Also, after 1993, the fireproofing was thickened from 1.3 cm (0.5 inches) to 3.8 cm (1.5 inches). The protection was thickened on several floors:
1 WTC
31, 35, 36, 37, 58, 59, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 108
2 WTC
14, 15, 17, 32, 40, 50, 56, 77, 78 (partially in the strike zone), 85, 88, 89, 91