(CNN) – A survey of rainfall in Denver and Boulder, Colorado has found that rain contains microscopic pieces of plastic.

The rainfall survey, titled “It is raining plastic,” was put together by scientists at the US Department of the Interior and US Geological Survey.

Scientists say you cannot see the plastic with the naked eye, but it can be seen using a binocular microscope fitted with a digital camera.

They found plastic showed up in 90% of the samples, mostly in fiber form, and came in a variety of colors. The study concluded that blue was most prevalent, followed by red, silver, purple, green, yellow and other colors.

It is unclear where the plastic is coming from, but plastic contamination has been a growing problem throughout the world.

Microplastic particles in rain have been found before in the Pyrenees in southern France. It has also been found in remote and otherwise pristine islands.

Trillions of pieces of plastic litter float through the ocean, killing fish and other animals. An earlier study found that people are swallowing an average of 5 grams of plastic every week, about the weight of a credit card.

Scientists say it is unclear what the health effects of living with all this plastic will be, although several studies are underway.

Some cities have tried to cut back on plastic by banning plastic bags. Boulder, Colorado is one of the sites for the rain survey, and has a plastic bag ban.