The main process building surpassed The Pentagon as the largest building in the world, with a floor area of , and an enclosed volume of . Construction required of concrete, and of gas pipes. Because uranium hexafluoride corrodes steel, and steel piping had to be coated in nickel, smaller pipes were made of copper or monel. The equipment operated under vacuum pressures, so plumbing had to be air tight. Special efforts were made to create as clean an environment as possible to areas where piping or fixtures were being installed. J. A. Jones established a special cleanliness unit on 18 April 1944. Buildings were completely sealed off, air was filtered, and all cleaning was with vacuum cleaners and mopping. Workers wore white lintless gloves. At the peak of construction activity in May 1945, 25,266 people were employed on the site.