A year into the coronavirus pandemic, buying a heavy-duty medical mask online remains downright maddening.

The most coveted mask to keep safe against Covid-19 has been the N95, the gold standard for pandemic protection because of its tight fit and 95 percent efficiency in filtering airborne particles. Then there’s the KN95 from China, a mask for medical workers, which also offers high filtration and is somewhat looser fitting.

But these masks have been far from easy to buy on the internet. When the pandemic hit last year, they immediately became scarce as health care workers and governments rushed to obtain them. The demand was so intense that a gray market sprang up for them.

Yet even after supplies have improved, it is often not easy to find authentic N95s and KN95s online. That’s because there are few brand-name makers, so it can be hard to know which of the dozens of manufacturers are reliable. And counterfeiters continue to flood the market, even on trusted sites like Amazon.

The result is frequently frustration, when wearing a heavy-duty mask is more important than ever. Last week, federal health officials emphasized the need for all of us to have tightfitting masks because of new fast-spreading coronavirus variants.

“People don’t know what’s legit, and they don’t know which suppliers are legit,” said Anne Miller, an executive director of Project N95, a nonprofit that helps people buy protective coronavirus equipment. “We’ve had that issue since the very beginning of the pandemic.”

I recently spent hours comparing masks online and almost bought a pack of counterfeits on Amazon. Thankfully, I avoided falling into the trap and eventually found legitimate, high-quality masks from a trustworthy online retailer.

Along the way, I learned plenty about how to spot fraudulent mask listings and how to sidestep fake reviews. So here’s how to home in on real medical-grade masks that will keep you and your loved ones safe.

Pick a mask

My journey began on the website for the Centers for Disease Control and Prevention. There I found charts of N95 and KN95 masks that the agency has tested, including the make, model number and filtration efficiency.

After some reading, I learned about the trade-offs between the two types of masks. The N95s typically have bands that strap over the back of your head, which is what makes them snug. They can be uncomfortable to wear for long periods.

The KN95s, which the Food and Drug Administration has approved for emergency use by health care workers, have ear loops for a tight fit that is slightly more comfortable than an N95. The downside is that the KN95 leaks a bit more air than an N95.

If you are often in high-risk areas like hospitals, N95s may be more suitable. But if you just need a protective mask for more casual use, like the occasional trip to the grocery store, KN95s are probably sufficient.

After doing the research, I decided a KN95 mask from Powecom, a Chinese brand, was best for my purposes. The mask scored 99 percent filtration efficiency in the C.D.C.’s tests.

From there, I visited Amazon, where I buy everything from dog food to batteries in the pandemic. That’s when things went awry.

Beware of Amazon

When I typed “Powecom KN95” into Amazon’s search box, the masks instantly popped up with a rating of 4.5 stars. I quickly clicked “Add to Cart.”

But before checking out, I scrolled down to read the reviews. There were about 130 — including a handful of one-star reviews from aggrieved buyers who said the masks were most likely fake. I emptied my shopping cart.

How had I almost bought a counterfeit? Saoud Khalifah, the founder of Fakespot, a company that offers tools to detect fake listings and reviews online, said a third-party seller had probably taken control of the product listing and sold fakes to make a quick buck.

“It’s a bit of a Wild West,” he said. “The normal consumers that shop on Amazon do not know that they just bought a fake mask. This is the biggest critical problem: You think it’s real, and suddenly you get sick.”

Mr. Khalifah presented other examples of questionable masks that were being sold on Amazon:

• A pack of 50 masks was highlighted on Amazon this week as the No. 1 new release in women’s fashion scarves. Obviously, masks are not scarves, which was a giveaway that something was off. The listing description also replaced all of the letter A’s with accented characters. This was a technique used to bypass Amazon’s fraud detection systems, Mr. Khalifah said. Amazon removed the listing after I called about it.

• Another pack of 20 masks looked attractive and was described as approved by the C.D.C. It even had positive reviews with an average of 4.4 stars. But the reviews revealed that most customers had received the masks for free, probably an incentive to leave positive feedback. One lukewarm review from someone who had paid for the product noted that the masks were “thin and very, very big.”

• Mr. Khalifah’s software also detected that the reviewers of another pack of 100 masks, which had unanimous five-star ratings, had a history of writing promotional reviews for other brands.

Amazon said in a statement that it prohibits the sale of counterfeit products and invests to ensure its policy is followed. It said it had specific policies for N95 and KN95 masks, including a process for vetting inventory and taking action on those who sold fakes.

Amazon also said it had addressed the questionable Powecom mask that I nearly purchased, as well as the mask advertised as a scarf. It added that there was no evidence that the pack of 20 masks was counterfeited and did not comment on the pack of 100 masks.

Mr. Khalifah cautioned that the fakes he spotted on Amazon could just as easily be on websites for other big retailers, such as Walmart and eBay, that allow third-party sellers to ship products. To buy authentic masks, he said, I should take a less traditional approach to shopping online.

Order from an authorized source

Armed with this advice, I continued my search for the Powecom mask.

I visited the manufacturer’s website, which listed steps for verifying that a mask is real. That involved scanning a bar code on the package with a phone camera. Then I did a web search for the mask, which brought me to bonafidemasks.com, an online retailer that shows documentation stating that it is an authorized distributor of Powecom masks in the United States.

That was more reassuring. So I ordered a pack of 100 for $ 99. When the package arrived in the mail, I scanned the bar codes to confirm their authenticity. They were the real deal.

Another path I could have taken was to order masks directly from the manufacturer. Verified mask producers like DemeTech, in Miami, and Prestige Ameritech, in Texas, sell N95s through their websites.

But ordering directly from a manufacturer presents other challenges. Often you have to buy a large quantity to reduce the cost.

So what if you just want to buy a few to try on? Ms. Miller’s nonprofit Project N95 buys bulk orders of masks and breaks them up so people can buy smaller batches. “It’s a very painstaking process to go through,” she said.

No kidding.