Our understanding of properly engineered HVAC systems was further advanced after the World Health Organization (WHO) issued a report in 1984 on Sick Building Syndrome (SBS). SBS was found to result from poor indoor air quality, and several solutions have since been developed to prevent this condition from developing. Proper maintenance guidelines for HVAC systems can help to prevent SBS; additionally, the use of an air-to-air heat exchanger can be employed to increase the amount of fresh outdoor air that is brought into a building without sacrificing energy efficiency. The current recommendation from American Society of Heating, Refrigeration & Air Conditioning Engineers is to provide 8.4 exchanges of air within a 24-hour period.