SBS can be caused by flaws in the heating, ventilation, and air conditioning (HVAC) systems. Other causes have been contaminants produced by outgassing of some types of building materials, volatile organic compounds (VOC), molds (see mold health issues), improper exhaust ventilation of ozone (byproduct of some office machinery), using light industrial chemicals in the building, or lack of adequate fresh-air intake/air filtration (see Minimum Efficiency Reporting Value).