(Remember the giant “fatberg” in London?)

Dental floss

Dental floss, which is usually made of nylon or Teflon, should also stay out of the toilet.

“It seems like, ‘Oh, it’s just a little string,’ but it tends to wrap things up,” Mr. Villée said. “It’ll collect other things and make kind of a big wad of stuff. It’s incredibly strong.”

Contact lenses

When contact lenses are flushed down the toilet or washed down the sink, they do not biodegrade easily. As a result, they may make their way into surface water, causing environmental damage, new research has shown.

[The science of why contact lenses may have a dark side if they are improperly disposed]

The lenses are also impervious to the bacteria that break down biological waste at treatment plants. When researchers at Arizona State University submerged contacts in chambers with the bacteria, they found that the lenses appeared intact seven days later.

“We discourage any kind of plastics because it can make its way through a treatment plant and end up in the receiving water,” said Vincent Sapienza, the commissioner of the New York City Department of Environmental Protection.

Contact lenses “have densities similar to water, so they don’t readily float or sink at wastewater treatment plants,” he added, which means they are not captured and removed at the plant.

Tampons

It is often assumed that tampons can be flushed down the toilet, in part because they are so small. But their absorbent materials, including the string, do not break down easily: They cannot be processed by wastewater treatment centers, and can damage septic systems.