DENVER -- Anyone who uses aluminum water bottles thinking they are free of the chemical bisphenol A -- or BPA -- may be surprised to learn that some are not.

The Swiss-manufacturer, SIGG, has issued a "voluntary exchange" for some of their pricey aluminum water bottles. That's because, despite claims they were environmentally safe, they contain trace amounts of BPA.

Even though the news surfaced several months ago, many people are just learning about it.

"I want a good bottle that doesn't leech anything into the water itself. That's why we bought them," said Tom Calhoun.

Calhoun has a few SIGG water bottles but since August, he's not been using them. That's because he heard about SIGG's admission that its popular bottles made before August 2008 contained small levels of BPA. He felt the company had misled its customers by not notifying the public sooner.

"Since the news has been revealing that there are issues here, I think the right thing to do is for a company to acknowledge that openly, make it public and inform people to do something about it," Calhoun said.

Professor Mark Anderson teaches chemistry at the University of Colorado Denver. He said the main concern is that BPA, a chemical believed to have links to heart disease, diabetes and some neurological problems, is an integral part of some aluminum bottle linings.

"It can upset the normal biological phenomena that occur in our body," Anderson said.

SIGG now admits that BPAs were found in their "proprietary epoxy resin liner."

While Anderson does not believe anyone is at risk because of these bottles, he does suggest not using those with the questionable lining.

"I think if a manufacturer issues a voluntary recall, I would definitely do it," Anderson said.

To find out if your SIGG bottle is the one impacted by the "voluntary exchange" unscrew the cap and look in the bottle. If the top of the bottle is bronze, it needs to be turned in. If it's bright yellow, you can keep the bottle.

You have until Oct. 31 to exchange your old SIGG bottles for the new ones. You can exchange your bottle by going through SIGG.com but you will have to pay a shipping fee.

Or you can go to Whole Foods, which is offering free exchanges for all customers -- no matter where you bought the bottles.

SIGG is now making BPA-free bottles.

Additional Resources:

Read SIGG's CEO response, admitting that SIGG knew about the BPA since 2006.
How to tell the difference between the old and new SIGG liners, go to MYSIGG.com/liner or MySiGG.com/bulletin.
To exchange your bottle through SIGG, go to SIGG.com