United States Environmental Protection Agency

OSHA
Occupational Safety and Health Administration, US Department of Labor

Thomas Register of American Manufacturers
The nation's largest industrial buying and specifying guide

SMACNA International
Resource to SMACNA Members and the sheet metal and air conditioning industry

ACGIH
Association of health and safety professionals devoted to protecting the health of the American worker and the environment

Air Movement and Control Association
Nonprofit trade association for manufacturers of air movement and control equipment, including fans, louvers, dampers and related air systems

Calculators
Martindale's Calculators On-Line

Knovel
Technical Engineering Reference Information

Engineers Edge
Mechanical Design, Manufacturing and Engineering Calculators