Buying a bicycle helmet, you might think it meets all the government safety standards required to protect your head. However, if you shop online, this is not necessarily the case.

A recent Consumer Reports survey found that potentially unsafe helmets — illegally sold in the United States — are “widely available” online.

"The helmets need to meet the standards of the Consumer Product Safety Commission (CPSC) to show that they can prevent skull fractures," Consumer Reports health reporter Kevin Loria said. "Part of the certification includes the posting of a label stating that it passed this test. But there are many helmets sold without these labels, so they may not be certified and should not be sold legally."

Consumer Reports (CR) purchased 13 different styles of helmets from four online markets without requiring CPSC labels: Amazon, Sears, AliExpress (a subsidiary of Chinese retail giant Alibaba) and LightInTheBox (headquartered in Beijing's global retailer).

Some helmets purchased by Consumer Reports comply with the European "CE" standard, which is different from the American standard. The US Consumer Product Safety Commission requires helmets to withstand greater impact.

Missing labels or incorrect safety labels make the helmet non-compliant, so it is illegal to sell in the U.S.

"CR does not mean that these helmets are absolutely unsafe, and it cannot be determined without testing them," Loria pointed out in his report on these findings. "But in fact, they are not affixed with the required label, which indicates that they may not have been tested according to the CPSC standard."

Dr. Fred Rivara, vice chair of the Department of Pediatrics at the University of Washington, is an injury prevention expert and has been researching bicycle helmets for more than 30 years.

"I am shocked that the helmets sold here in the United States do not meet the standards of the Consumer Product Safety Commission. This is crazy," Rivara told NBC News Better. "I'm also surprised that CPSC allows these things to be sold here."

The Consumer Product Safety Commission responded to Rivara's comments with the following statement:

"The Consumer Product Safety Commission has established mandatory safety standards for bicycle helmets to protect consumers. Manufacturers and importers must meet the standards. The law requires companies to test their helmets before they are sold in the United States to ensure that their helmets comply with CPSC mandatory Helmet safety standards. We will not test all products before they are on the market. When the US Consumer Product Safety Commission finds non-compliant products in ports, markets or online, we will take appropriate action."

CPSC regulations require the following information to be provided on the helmet label:

"Consumers should be aware that there is no label indicating that they meet the mandatory safety standards of the US Consumer Product Safety Commission for bicycle helmets. Helmets that do not meet the CPSC safety standards may not be able to protect your head in a collision," CPSC explained.

NBC News BETTER contacted Amazon, Sears, AliExpress and LightInTheBox regarding the findings of Consumer Reports.

Amazon, Sears and AliExpress stated that the helmets in question were sold by third-party companies, which are contractually obliged to comply with all relevant laws and regulations. The three websites stated that these helmets are no longer available. LightInTheBox did not respond.

The helmet manufacturers contacted by NBC News BETTER praised Amazon, Sears and AliExpress for taking action, but pointed out that there are too many unsafe helmets on the market, which will not cause any impact on what they call "very serious".

Bicycle helmets are important safety equipment; according to the American Academy of Pediatrics, they "prevent up to 88% of serious brain injuries."

Bicycle helmets are important safety equipment; according to the American Academy of Pediatrics, they "can prevent up to 88% of serious brain injuries."

Therefore, it is very important to purchase products that meet federal safety standards. If you buy a helmet online, you cannot check the safety certification label before buying.

The risk of obtaining counterfeit or generic counterfeit helmets is also high. The counterfeit product looks similar to the brand-name helmet, but does not have the logo and all the safety features designed in the genuine product.

These helmets are cheap and beautifully shaped, but they cannot protect your head in the event of a collision.

"We have never seen fakes or imitations that perform well in the laboratory," said Thom Parks, senior director of product safety at Bell Sports. "We have seen that some of the normal types of accidents may work normally, but they are usually only affected by the standard (required for certification), and it is easy to malfunction, and sometimes it is a major malfunction."

Andrew Love, the head of brand safety at Specialized Bicycles (which also produces helmets), focuses on getting these low-quality helmets off the shelves and educating consumers on what to pay attention to when shopping.

"I'm sure that those who wear fake or counterfeit helmets are dying," Love said.

Specialized has a counterfeit awareness section on its website that explains the problem and shows customers how to identify counterfeit products.

"When buying towels or socks or anything else, you can go online and find a lot of things," Love told NBC News BETTER. "But when it can protect your brain from being affected, I strongly recommend that you go to your local bicycle store or major established retailers, because in the United States, fakes will not enter their supply chain."

Another benefit of going to a local store is that you can make sure that the helmet fits. Inappropriate or improperly worn helmets will not provide adequate protection.

If you shop online, Consumer Reports recommends buying directly from the manufacturer or authorized seller or reputable retailer with online business, such as REI and Performance Bicycle.

The Consumer Reports Bicycle Helmet Buying Guide shows how to wear and wear a helmet correctly. You might also want to check out its best bicycle helmets of 2019. Three recent helmets failed the CR test and were rated as "Don't Buy: Safety Risk".

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Herb Weisbaum is a contributor to NBC News, writing consumer-related questions. He can be found on Facebook, Twitter or The ConsumerMan website.