Glass cookware generally including that manufactured and sold under the Pyrex brand is made from glass subjected to a tempering process that makes it stronger and more durable than ordinary glass. It's still susceptible to breakage due to mishap or mishandling, however, including droppage or exposure to sudden, extreme temperature changes ("thermal shock").
Pyrex cookware sold in the United States is made from tempered soda lime glass composite. In Europe, the same products are made from borosilicate glass. According to Trevor Butterworth of STATS.org, tempered soda lime glass is more susceptible to breakage via thermal shock than borosilicate. Borosilicate, on the other hand, is more susceptible to mechanical (impact) breakage.
The email is correct when it says that Pyrex products sold in the United States used to be made of borosilicate glass, but incorrect when it claims that World Kitchen, the current U.S. manufacturer of Pyrex, instituted the change. According to World Kitchen, the switch to soda lime glass composite was made 60 years ago when the brand was still owned by Corning. World Kitchen which is a U.S. company headquartered in Rosemont, Illinois, by the way, and not a foreign-based company as rumored has owned the brand and manufactured Pyrex products since 1998.
Manufacturer maintains that Pyrex cookware is safe
World Kitchen maintains that Pyrex cookware made from soda lime glass composite is perfectly safe when used as directed. A statement on the company's website notes that the U.S. Consumer Products Safety Commission has never recalled a single Pyrex product, which is true. In an interview with Chicago's WBBM-TV News last year, a CPSC spokesperson confirmed that the Commission does not regard Pyrex as a safety hazard.
The 66 American consumers who filed complaints about Pyrex with the Commission over the past 10 years presumably disagree, although, again, a total of 66 complaints compared to 370 million units sold during roughly the same time period constitutes a very small fraction indeed, and in fact confirms the overall safety of the products.
The important take-away is this: Though rare, non-mechanical Pyrex breakages do sometimes occur. The probable cause is thermal shock caused by extreme temperature changes, such as when a hot dish is placed on a cold or wet surface, or directly on a stovetop burner. Mechanical breakages can occur through mishandling as well, and are more likely to occur if the cookware is chipped or cracked. If you use Pyrex cooking products, do read the directions.
Jan. 2011 update from Consumer Reports magazine:
• Glass Bakeware that Shatters: We Put Pyrex and Anchor Hocking Dishes to the Test
Sources and further reading:
Pyrex Use and Care
Pyrex Panic: Shrapnel in the Kitchen
ConsumerAffairs.com, 27 March 2006
Three Years Later: Pyrex Dishes Still Go Boom
ConsumerAffairs.com, 20 August 2008
World Kitchen & Pyrex Respond to ConsumerAffairs.com Claims
ConsumerAffairs.com, 8 October 2008
A Problem with Pyrex
CBS News, 28 February 2008
Why Pyrex Bowls 'Explode'
The Consumerist, 30 January 2007
CBS Sweeps Week Shocker: Glass Can Break!
STATS.org, 26 February 2008
Pyrex-O-Mania Continues on CBS Chicago
STATS.org, 27 February 2008
Does Pyrex 'Explode' Because the Manufacturer Change the Mix?
STATS.org, 28 February 2008
Pyrex, Soda Lime, Borosilicate, and the Environment
STATS.org, 29 February 2008
Last updated 11/09/12