Compared with the obvious environmental issues we hear about every day, littering often takes a backseat-but it’s more pressing than we may think.

Some may say that a banana peel out of your car along the motorway would be a harmless action. Actually, they are wrong. A banana peel can take up to two years to decompose(分解)，and with a third of motorists admitting to littering while driving, that’s a whole lot of discarded banana peels, or much worse. An orange peel and a cigarette butt has a similar biodegrading(生物降解)term to that of a banana, but tin and aluminium cans last up to 100 years, and plastic bottles last forever, so do glass bottles and plastic bags.

Despite the fact that longer-lasting materials will serve to damage the environment and its animals for longer, we can’t only measure the severity of a certain type of rubbish by its lifetime. For example, despite having a fairly short biodegrading span, more than 120 tons of cigarette-related litter is thrown away in the UK every day. Similarly, our regular littering here and there has caused the UK’s mouse population to increase by 60 million. This suddenly isn’t so mysterious when you consider that since the 1960s our annual littering has increased by an amazing 500%.

It’s not a cheap habit either: UK taxpayers spent $500 million on keeping the streets clean. So, it’s not surprising that if caught fly-tipping, you could face a $20, 000 fine. Regardless of how severe the punishment might seem, however, among the reported cases only 2, 000 were punished out of 825, 000, so we still have some way to go in making sure people observe the rules.

To take back our beautiful cities, we need to do more than simply not leaving rubbish where it ought not to be. We need to care more about the world around us.

People often recommend planting trees to make cities greener, cleaner and healthier. But during heat waves, city trees can actually increase air pollution. Indeed, a new study has found, up to 60 percent of the smoggy ozone in a city’s air on hot days may trace to chemicals emitted by trees. Galina Churkina, who works at Humboldt University of Berlin, and her team have confirmed it.

The findings might seem the opposite of what you would expect, notes Robert Young, an expert in city planting at the University of Texas at Austin. Indeed, he says, “Everything has multiple effects.” The new findings do not mean cities should discourage tree planting. Instead, cities may need stricter controls on other sources of pollution, such as tailpipeemissions from cars and trucks.

City trees soak up carbon dioxide, and at the same time they release oxygen into the air. But oxygen is far from the only gas that trees and certain other green plants release into the air. One of these chemicals is a hydrocarbon known as isoprene(异戊二烯). It can react with combustion(燃烧)pollutants, such as nitrogen oxides(氮氧化物)emitted by cars and trucks in cities. The result is the formation of ozone, a component of smog, which can irritate the lungs and cause airway diseases.

Churkina says her team is not surprised to see the seemingly contrary relationship between plants and pollution. She adds that its importance is quite amazing. “The results,” Churkina says, “suggest that city tree planting programs should not ignore the role this greenery may play in aggravating summer air pollution.” “Adding more trees will improve quality of life only if those cities also undertake plans to sharply cut vehicle pollution in summer and to increase their reliance on clean energy sources for electric power,” she says.

Carried by the wind, dust particles (微粒) from places such as the Sahara Desert can float halfway around the world before settling to the ground. As the plastics abandoned by humans break down into tiny pieces in the environment, they, too, travel through the atmosphere. Now scientists are a step closer to understanding how these microplastics travel in the globe — both locally and on long-distance flights.

Researchers spent more than a year collecting microplastics from 11 national parks and wilderness areas in the western U.S. They examined the particles that settled on dry days and those that fell along with rain or snow. In addition to making clear how microplastics move around, the results, published on Thursday in Science, reveal the seriousness of the problem: more than 1 million kilograms of microplastics — the weight of 120 million to 300 million plastic water bottles — fall on protected lands in the country’s western region each year.

The new findings add to scientists’ concern over microplastic pollution’s potential impacts on the environment and human health. “We’re not supposed to breathe in this material,” says Steve Allen, a microplastics researcher at the University of Strathclyde in Scotland, who was not involved in the new study. “Plastics in the environment “carry all sorts of pesticides (农药), heavy metals and all the other chemicals that we’ve made over time,” he adds. “They’re going to carry them directly into our lungs.”

Since their discovery in oceans in the 1970s, microplastics — which can be as large as a grain of rice or smaller than a particle of dust — have been found nearly everywhere researchers have looked: in cities, in Arctic snow, on remote mountaintops. Their presence in areas distant from the place where human live has pointed to them being carried by winds.