BOSTON -- Federal law requires companies to install tempered safety glass in bathrooms, patio doors, automobiles and skylights. But even a nick tinier than a grain of sand can cause the panes to suddenly shatter.

"The house shook and we heard just crashing noises," said Jennifer Day after the glass door installed just three years ago in her shower exploded inexplicably.

"You could tell it didn't just break and fall," Day said. "It literally exploded for no apparent reason."

Luckily, no one was hurt, but the family was stunned.

"I never would have thought tempered glass could explode," Day said. "I think of tempered glass as safety glass."

Dennis Campeau at East Coast Glass installed the Day's shower and thousands more like it. He said in 25 years, he's never seen this happen.

"I could break a baseball bat on that glass," Campeau said. "I know it broke. There's no way of knowing how it broke. I've never seen one break, so I can't explain it."

But tempered glass can -- and does -- break. And the pieces can be sharp enough to cause injuries.

"It can be a flaw in the glass," said Rick Shaw of Solar Seal, a local glass manufacturer. "And when I say flaw, we're looking for something in a piece of glass smaller than a grain of sand."

Shaw said even flawed glass may not shatter for years.

Team 5 Investigates has already reported on incidents involving tempered glass where glass cookware and patio tables exploded for no obvious reason. In response, last winter, doctors at Children's Hospital issued a report and warning against using glass tables in homes where toddlers reside.

"I think people need to know that it is a possibility," Day said. "That it could happen."