Conspiracy theories are as American as apple pie. A national poll released this week by Public Policy Polling (PPP) found that some popular conspiracy theories in the United States have persisted for years, like the belief that a UFO crashed in Roswell, New Mexico, in 1947.

Meanwhile, new theories have gained believers, such as the ideas that Saddam Hussein was involved in the 9/11 attacks and that Osama bin Laden is still alive.

Perhaps not surprisingly, many of the ideas in the new poll are based on a misunderstanding of science—or a stubborn refusal to acknowledge it.

Here's a reality check on some of the science-related conspiracy theories from the survey:

1. A total of 37 percent of American voters believe global warming is a hoax. Most (58 percent) of the people who believe this identified themselves as Republicans. Of those who don't believe global warming is a hoax, 77 percent are Democrats.

Reality Check: Numerous scientific studies have confirmed that the Earth is warming and that the rate of warming is increasing. Average temperatures have climbed 1.4°F (0.8°C) around the world since 1880. Much of the temperature increase happened in recent decades, coinciding with a spike of carbon dioxide and other greenhouse gases emitted into the atmosphere as a result of human activity.

The effects of global warming on the natural world are equally well documented: Arctic sea ice is now thawing at historic rates, flowers are blooming earlier, and the migration patterns of birds and other animals are changing.

2. A total of 29 percent of voters believe aliens do exist. Another 21 percent believe the U.S. government covered up a UFO crash near Roswell, New Mexico, in 1947.

According to PPP, more Americans who supported Mitt Romney for president last year (27 percent) than those who supported Barack Obama (16 percent) believe in the UFO cover-up.

Reality Check: It hasn't always been the case, but many astronomers today are open to the idea of life existing elsewhere in the universe—and even to the possibility of intelligent alien life. That's thanks in large part to the relatively recent discoveries of hundreds of so-called exoplanets—worlds beyond our solar system—and thousands of planet candidates. Most scientists think it's only a matter of time before a habitable, rocky, Earth-like twin is discovered.

Even our own solar system might contain evidence of alien life. NASA's Curiosity rover recently discovered tantalizing evidence of clays and minerals that scientists say could only have formed in water. The implication: Ancient Mars had the conditions and ingredients necessary to support life.

As for UFOs, most sightings are eventually linked to more mundane causes. For example, a recent video by actor Russell Crowe purportedly capturing a UFO outside his office was likely reflected sunlight from a plane passing near sunset.

3. Some 20 percent believe childhood vaccines are linked to autism. These voters believe that childhood vaccines against mumps and other diseases could increase the risk of developing autism.

Reality Check: A recent government study confirmed what many scientists have been saying all along: There is no connection between the number of vaccines a child receives and his or her risk of developing autism.

The study, published last month in The Journal of Pediatrics, also found that even though kids are getting more vaccines these days, those vaccines contain fewer substances that provoke an immune response.

4. The poll revealed that 14 percent believe that Bigfoot is real. Another 14 percent said they were not sure, while 72 percent said they do not believe Bigfoot is real.

Reality Check: Despite several attempts to prove Bigfoot exists, no one has presented evidence that has withstood scientific scrutiny. Indeed, many such "proofs" have turned out to be outright hoaxes. In 2008, two men claimed to have found a seven-foot (two-meter) tall, 500-pound (230-kilogram) Bigfoot corpse in the woods of northern Georgia, but the body was later revealed to be a rubber ape costume.

Last November, another group claimed they had done DNA tests that proved the "North American Sasquatch is a hybrid species, the result of males of an unknown hominin species crossing with female Homo sapiens." The researchers touted the fact that their study was published in a scientific journal called DeNovo—but it seems the publication was created especially for that Bigfoot study.

While Bigfoot is likely just a myth, that's not to say that no new, close relatives of humans have ever been found—it's just that all of them are long extinct. For example, in 2010, scientists announced that a 40,000-year-old pinkie bone found in a Siberian cave belonged to a previously unknown species of ancient human called Denisovans.

5. Some 9 percent believe the government adds fluoride to drinking water for "sinister" purposes.

Reality Check: The latest evidence that fluoridated water has dental health benefits comes from a 2013 study published in the Journal of Dental Research. The study found that fluoride in drinking water prevents tooth decay in adults regardless of age, whether or not they drank fluoridated water as children.

Other recent evidence of the dental benefits of fluoride came from an unlikely source: A survey of more than 23,000 skeletons from medieval archaeological sites in Britain showed that people who lived near the coast—and presumably consumed a lot of fluoride-rich fish—had fewer cavities.

6. A total of 7 percent of voters believe the moon landing was faked. Another 9 percent said they weren't sure whether the Apollo moon landing really happened.

Reality Check: Believers of this particular conspiracy theory have painstakingly dissected video and photos from the Apollo 11 moon landing looking for evidence that it was faked. For example, some have pointed out that the American flag Neil Armstrong planted on the moon appears to be flapping "in a breeze" in videos and photographs.