Drywall imported from China during the homebuilding boom years of 2004 and
2005 was made with waste materials from scrubbers on coal-fired power
plants.

Those materials leak into the air as gases and combine with the moisture on
an air conditioning coil to create sulfuric acid, which appears to be
dissolving solder joints and copper tubing - creating leaks, blackening the
coils and even causing the system to fail.

http://www.news-press.com/article/20.../81220026/1076

Best Regards
Tom.