How Long Does It Really Take For A Plastic Bag To Decompose?

Dear EarthTalk: I’ve heard conflicting reports regarding how long it really takes for a plastic grocery bag to decompose. Can you set the record straight?

—Martha Blount, San Diego, CA

Researchers fear that such ubiquitous bags may never fully decompose; instead they gradually just turn into smaller and smaller pieces of plastic. The most common type of plastic shopping bag is made of polyethylene, a petroleum-derived polymer that microorganisms don’t recognize as food and as such cannot technically “biodegrade.” The U.S. Environmental Protection Agency defines biodegradation as “a process by which microbial organisms transform or alter (through metabolic or enzymatic action) the structure of chemicals introduced into the environment.” In “respirometry” tests, whereby experimenters put solid waste in a container with microbe-rich compost and then add air to promote biodegradation, newspapers and banana peels decompose in days or weeks, while plastic shopping bags are not affected.

Even though polyethylene can’t biodegrade, it does break down when subject to ultraviolet radiation from the sun, a process known as photodegradation. When exposed to sunshine, polyethylene’s polymer chains become brittle and crack, eventually turning what was a plastic bag into microscopic synthetic granules. Scientists aren’t sure whether these granules ever decompose fully, and fear that their buildup in marine and terrestrial environments—and in the stomachs of wildlife—portend a bleak future compromised by plastic particles infiltrating every step in the food chain. A plastic bag might be gone in anywhere from 10 to 100 years (estimates vary) if exposed to the sun, but its environmental legacy may last forever.

According to the Worldwatch Institute, Americans only recycle 0.6 percent of the 100 billion plastic bags they take home from stores every year; the rest end up in landfills or as litter. Pictured: An anti-plastic bag activist makes a point in Austin, Texas.© Ret0dd, courtesy Flickr

The best solution to plastic bag waste is to stop using disposable plastic bags altogether. You could invest a few bucks in reusable canvas totes—most supermarket chains now offer them—or bring your own reusable bags or backpacks with you to the store. If you have to choose between paper and plastic, opt for paper. Paper bags can biodegrade in a matter of weeks, and can also go into compost or yard waste piles or the recycling bin. Of course, plastic bags can be recycled also, but as just explained the process is inefficient. According to the nonprofit Worldwatch Institute, Americans only recycle 0.6 percent of the 100 billion plastic bags they take home from stores every year; the rest end up in landfills or as litter.

Another option which some stores are embracing—especially in places like San Francisco where traditional plastic shopping bags are now banned in chain supermarkets and pharmacies—are so-called compostable plastic bags, which are derived from agricultural waste and formed into a fully biodegradable faux-plastic with a consistency similar to the polyethylene bags we are so used to. BioBag is the leader in this field, but other companies are making inroads into this promising new green-friendly market.

San Francisco’s pioneering effort to get rid of polyethylene bags is a positive step, but environmentalists are pushing for such bans more widely. A California effort to ban plastic bags failed again recently, but will likely eventually succeed. Washington, Florida, New Jersey and North Carolina are watching closely and considering similar laws depending on what happens in the Golden State. Worldwatch reports that taxes on plastic bags in South Africa and Ireland have been effective at reducing their use by upwards of 90 percent; Australia, Canada, New Zealand, the Philippines, Taiwan and the UK are also planning to ban or tax plastic bags to help stem the tide of plastic waste.

CONTACTS: Worldwatch; BioBag.

Dear EarthTalk: What would it take to produce “green” tires? The tire industry is huge and I understand that tires contain a large amount of petroleum products. Is there an alternative?

—Scott Pierson, Norwalk, CT

Most tires are just one-third natural rubber and two-thirds petroleum and “carbon black” filler derived partially from burned fossil fuels. They also contain chemical additives to improve functionality. Once they’ve outserved their usefulness they are often burned, sometimes as fuel to power electric facilities, cement kilns, pulp and paper mills and industrial boilers, releasing benzene, lead, butadiene, styrene and other potential carcinogens into the air we breathe.© Damon D’Amato, courtesy Flickr

Tires are indeed no friends to the environment. Most tires on the road today are constructed of roughly equal parts natural rubber, petroleum and “carbon black” filler (derived partially from burned fossil fuels), along with a dash of other chemical additives to improve functionality. The tire industry has embraced recycling in recent years, but still some 25 percent of tires wind up in landfills, according to Michael Bloch of the website. Still others are incinerated, which releases benzene, lead, butadiene, styrene and other potential carcinogens into the air we breathe.

Even worse, Bloch reports, nearly half of the spent tires in the U.S. are used as “Tire Derived Fuel” (TDF) and burned alongside other dirty polluting fuels such as coal. According to the Rubber Manufacturer’s Association, old tires fuel cement kilns, pulp and paper mills and industrial boilers, and are used as well by electric utilities and some dedicated tires-to-energy facilities.

Beyond the actual ingredients in tires, environmentalists have also been critical of the tire industry for producing tires that stick to the road so well that they cause engines to burn extra fuel to overcome the added friction, which leads in turn to more greenhouse gas emissions out of our tailpipes.

In response to such criticism, tire engineers have begun incorporating a wide range of new materials as substitutes for petroleum and chemical fillers. Today consumers can already buy low rolling resistance tires that generate about five percent less friction than traditional tires. This translates into a four to eight percent boost in fuel economy depending upon the engine, according to Forrest Patterson of Michelin North America. Over a tire’s lifetime, that could save up to 80 gallons of gas, he says. Encouraging motorists to keep their tires inflated to proper levels has also helped reduce tire-related emissions.

What’s in these greener tires anyway? Chemically toughened natural rubbers, vegetable-based processing oils, and fibers made of plant cellulose are used to replace some of the petroleum in the newer so-called “low-oil” tires. Meanwhile, environmentally benign silica filler (sand microparticles) has been used to replace some of the carbon black reinforcement, with the added benefit of further reducing road friction.

Japan-based Yokohama Tire now sells what it calls the dB Super E-spec car tire, which employs modified natural rubber compounds and processing oil derived from orange peels in place of much of the petroleum in traditional tires. (The company likes to brag that the dB Super E-spec is 80 percent petroleum-free.)

While greener tires are already available, tire makers have been re-doubling efforts to recycle old tires into new ones to further reduce the industry’s environmental impact. Small quantities of reprocessed rubber are showing up increasingly in new tires, but manufacturers would like to see more of the 75 million or so tires Americans send to landfills get reprocessed to live another useful day as new tires or other products.

CONTACTS: Green Living Tips; Michelin North America; Yokohama Tire.

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