When Jeremiah Mock collected an entire ziplock bag of discarded JUUL pods from a Marin County high school’s parking lot, he considered an overlooked problem from increasingly popular e-cigarettes.

How do people throw them away?

“Like a cell phone or like a cigarette butt?” asked Yogi Hendlin, a postdoctoral scholar at the University of California San Francisco’s Center for Tobacco Control Research and Education.

Hendlin and Mock, an affiliated faculty member at the UCSF Institute for Health and Aging, are conducting a pilot study to figure that out. So far, the answer seems to be “cigarette butts,” but this is worrisome. Discarded e-cigarettes contain toxic chemicals that can leach into water and are combustible if not handled carefully. But despite growing evidence of their danger, neither the companies producing the vapor pens, nor agencies responsible for waste management are coming up with a good way to deal with the source of trash.

Cigarette butts themselves are an enormous environmental problem. In the U.S., they make up an estimated 30 percent of the total number of littered items in waterways, shorelines, and on land. The butts are made of plastic fibers that do not biodegrade, and they contain toxic chemicals such as arsenic, nicotine, and heavy metals that can leach into water, kill small animals like plankton and water fleas, and be mistaken for food by larger ones.

But when people toss spent e-cigarettes on the ground, they pose another set of environmental dangers.

E-cigarettes, handheld electronic devices that heat up liquid nicotine to form a smoke-free, tar-free vapor, are made up of five parts: a mouthpiece, a cartridge or “pod” that holds the liquid, a vaporizing chamber, a microprocessor, and a lithium-ion battery.

If damaged or exposed to extreme heat, the lithium-ion batteries in e-cigarettes can explode. The batteries have triggered several fires in garbage trucks and processing facilities across the Bay Area in the past few years, including two in Oakland last year, several in San Francisco, and one particularly large one at Rethink Waste, a garbage facility in San Carlos, in 2016. The company is still working to repair the damage to their electrical systems.

Brad Drda, Regional Environmental Manager at Recology, the garbage collection service in San Francisco, says lithium-ion batteries pose the biggest risk in waste facilities, where they can be crushed and damaged by large tractors and machinery.

According to California law, lithium-ion batteries, like those in e-cigarettes, contain material classified as household hazardous waste and cannot be thrown in the garbage.

But companies that accept used electronics like cellphones and computers won’t take e-cigarettes, because their small size makes them disadvantageous for recycling.

Meanwhile, the microprocessors in e-cigarettes are made with several harmful chemicals, including lead and mercury, which can potentially leach into water and soil, becoming toxic to humans and wildlife.

But again, drop-off hazardous waste facilities won’t accept e-cigarette waste because they have to be taken apart to remove the batteries for recycling. According to several YouTube tutorials, that’s a multi-step process involving a screwdriver and one’s mouth.

Despite these known hazards, there are no clear guidelines or regulations governing disposal of e-cigarettes and nicotine pods in California. E-cigarette makers do not offer instructions. One popular disposable e-cigarette brand, Blu, contains a lithium ion battery yet advises consumers on its website that they can be thrown in the garbage.

The state has yet to help either.

The California Department of Toxic Substances Control, the government agency that decides what is considered toxic or hazardous and enforces the proper handling of such materials, said in an email that they are not currently testing e-cigarettes and could not answer questions about them.

“This is an example of a kind of product that just showed up, and obviously not a whole lot of thought was given to its end-of-life,” said Drda.

In an effort to determine the scale of the e-cigarette litter problem, Hendlin and Mock are collecting tobacco waste from twelve high schools across Marin, Alameda, Contra Costa and San Francisco counties. They are not yet sharing the names of the schools, Mock said.

Prior studies have shown toxicity varies across brands.

A 2015 University of Florida study tested the toxicity of 15 different types of e-cigarettes. The e-cigarettes, consumed of their nicotine, were ground up into tiny particles, put in an environment that simulated a landfill, and analyzed to determine how much of the hazardous material “leached out” of the waste. The study found that some spent e-cigarettes have the potential to be constituted as hazardous waste under U.S. law, though the majority tested did not.

There’s another wrinkle to the e-cigarette disposal problem: some e-cigarettes are disposable; others are re-chargeable. Disposable e-cigarettes, made for one-time use, create more waste than their rechargeable counterparts.

Among Bay Area teens, at least, rechargeable JUULs seem to be by far the most popular brand, according to Mock and Hendlin.

JUUL does not offer a recycling program for the disposable plastic cartridges that hold the nicotine, called pods, but users have found their own way to recycle them. Some people refill their own used pods; others buy used pods and refill them with off-brand e-juice.

“It is the fiscally responsible thing to do,” said someone on a Reddit forum where people sell empty used pods in bulk.

But Hendlin said refilling pods with e-juices that contain varying percentages of nicotine is a risky proposition for teens to be undertaking “with this very dangerous stuff.”

Nicotine spilled on the skin can cause nausea, cardiac problems, seizures and, in severe cases, overdose.

And less than a tablespoon, if swallowed, can kill an adult.