Medical Examiner

The Creepy Test Subjects Used in Mask Studies

A head form wearing a mask with a tube coming out of it.
CDC/YouTube 

The Centers for Disease Control and Prevention has a secret star research subject: a disembodied rubber head with fleshy eyes.

These “pliable elastomeric head forms” were featured in the CDC’s hot new paper on the benefits of wearing two masks at a time. In the study, the heads were attached to a machine that spewed aerosols from their mouthpieces and into different face coverings. The results of the research are clear: Double masking works. If two pliable elastomeric head forms are each wearing two masks, they hardly exchange any aerosols at all.

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We wanted to know more about those horror movie–esque dolls. Slate spoke with Chris Brown, a senior engineer at INSPEC, which is a global personal protective equipment test equipment supplier based in England and the makers of heads similar to those used in the double-mask study. (The only difference: The ones used in the study have ears, while INSPEC’s do not.)

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Brown told us about ways to make these heads look even more creepy, as well as why his company’s current design of disembodied rubber head is on its way out the door. Our conversation has been edited and condensed for clarity.

Elena DeBré: In the CDC study, the heads spray out aerosols, to simulate a person breathing. How do the heads do that?

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Chris Brown: Head forms themselves don’t do anything. It’s just a mock-up of a person’s head. But the head is hollow, and it has a tube assembly that comes out of the mouth. Depending upon the test you are doing, you can attach the head to various machines, such as a breathing machine for respiratory tests. From there, you can inject CO2 or other gases into the machine themselves. The National Institute for Occupational Safety and Health bought heads from us many, many years ago. And they’re free to do what they want with it.

What is each head made of?

They tend to be an aluminum skull that is then covered with a half-inch of rubber on top to simulate the skin.

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Your heads have eyes without pupils. Do all head forms have the same facial features—or lack thereof?

It depends on what task you are doing with the head. If you’re conducting a respirator test, all you’re interested in is what’s coming in and out of the mouth. So, the eyes and ears will have some shape but be irrelevant. However, if you are testing a respirator that covers the whole of the face—like a face shield visor—and you want to study how much a person can see with the equipment on, then the eyes of the head form will be more detailed. They’ll be replaced with lightbulbs that you can hook up to another piece of equipment and use to test the eyes. Or if you’re using hooded PPE and testing the sound that can travel underneath it, the ears will be replaced with microphones. It depends on the task that you’re doing and what features are relevant. A lot of customization is available.

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Are there ever design malfunctions when producing the heads?

Yes. Sometimes the head has skin problems. If contaminants get into the rubber when you mix it, the application of the skin gets wrinkled and messed up. It will look like a tree has been tattooed onto the face. We’ve got loads of those around here.

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Are your heads available to the public?

If you can afford it. Each head runs a price that’s around 2,000 pounds [almost $3,000]. A lot of work goes into each one.

How many do people typically buy in each order?

A researcher will typically buy one. We recommend that a lab buys three or four of them just to ensure that they can do every test all day every day.

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In COVID times, with all the PPE research going on, has their popularity skyrocketed?

Definitely. At the beginning of last year, when the pandemic was starting to take hold, our company as a whole saw demand quadruple. For Sheffield heads specifically, we were having to produce 10 times the normal amount.

Your head forms, which have been used in past CDC studies on PPE, are specifically called “Sheffield heads.” Why is that?

Around 1988, researchers here in England looked at some anthropometric data and decided they needed to create a new head form standard. They didn’t need to search far: Someone in the Sheffield Health and Safety Office had a head the exact right size for it. So, they took a mold from his face. All Sheffield heads produced since have been made from the exact same mold of this head

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Wow. Can you tell me more about this guy?

This guy was a large, white male in his mid-40s. He required a pretty big mask to cover his face. Sheffield heads are used to batch-test masks. One out of every 1,000 masks produced in Europe are tested on a Sheffield head [as a quality control measure]. As a result, we used to produce quite large masks. Masks tended to always be oversized here.

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Isn’t that problematic, if PPE is being designed to fit this specific guy’s head?

It was never really an issue since the people who tended to wear PPE were typically large males. Firefighters or construction workers. But over the last 15 years, as these professions have become diversified, it’s really highlighted the fact that PPE is in fact the wrong size. Especially for women. Now a lot of PPE is made in China. The Chinese have their own face molds, which are smaller.

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Has your company changed with the times?

Going forward, we at INSPEC are going to have to change our head form standards to cover a larger range of the population. Historically—whether for developing respirators or helmets—the data that’s defined standard head forms came from molds of males in the 1940s or 50s, particularly from army personal. New and different head form sizes have been created in recent times, but the national standards [in Europe] haven’t yet changed. Once more countries adopt standards that encompass different sizes, we will start making different types of heads.

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What does that new, inclusive standard look like?

The international standard, or ISO, includes five different head sizes meant to represent 90 percent of the world’s population. It includes a wide range of ages, male and female heads, and different ethnicities. This is important since even within Europe, there’s not just white people. So, we have to adapt.

What is the future of the Sheffield head?

Probably in about five years, the Sheffield head will be ripped out of the standard. And we will all be working with the ISO head forms and producing quite a lot of them.

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