Is Dust Mostly Dead Skin?

This is me at the end of college. So anyway, today I'm packing up my room. It is absolutely disgusting. There’s dust all over the place. Unbelievable how much dust this place accumulates; just unreal. 21-year-old me was apparently fascinated by dust, but where does it all come from? Well, in this video, we are going to get to the bottom of this.

Now, this video was sponsored by Google, and when they approached me about sponsoring a video, I suggested I show you how I use Google to research a complex question. After all, research is just research anyway. I couldn't pick a simple question like how far away is the Moon, how old is Derek Muller, or how much wood could a woodchuck chuck if a woodchuck could chuck wood because the video would be over already. But in the space of all possible queries, questions like these with clear and ambiguous answers are by far in the minority. In fact, of all the billions of search queries typed into Google every day, fully 15% are ones that Google has never even seen before.

So, Google never actually gives you answers. Instead, in just a fraction of a second, it uses algorithms to select the most relevant information from the hundreds of billions of web pages it has indexed. What you do with that information is up to you. For this video, I intentionally selected something surrounded by mist: dust, unbelievable.

A while back, when I was filming a different video, a PhD scientist told me, “You know, 70% of household dust is dead skin.” I’d heard that before, and chances are you have too. If you type "is dust" into Google, the top autocomplete result is "dead skin," because that's one of the most common ways people complete that query. But a quick scan of this page suggests that the claim is not true; it's a misconception from Live Science.

Sometimes, a specific percentage of dust is said to be dead skin, usually about 70% or 80%. But unless you're a molting bird or a reptile or you work in Dr. Frankenstein's laboratory, very little of your environment is composed of dead body parts. BBC Science magazine, which asks "What is dust made of?", thinks it’s human skin. Think again! The article says it’s a misconception that it's mostly from outside. The rest is carpet fluff and clothes fibers. Sure enough, all over the web, you'll find people debunking the idea.

So, is that it? Case closed? Hardly! I mean, I'm not just gonna do one search and call it a day. It's important to be aware of your own biases. When I started this search, the claim seemed false. The idea that 70% to 80% of dust is dead skin is exactly the sort of thing that is gross enough to spread as an urban legend, but it just seems implausible. I mean, if most dust is dead skin, then why do abandoned buildings get dusty over time?

The debunking claims fit my preconceptions, so it would be easy to stop here. But you've got to be careful not just to search to confirm what you already thought. A common mistake people make is putting the answer they are looking for right in the search query. Looking more closely, these websites lack scientific papers as references, and I'm curious if the idea that 70% to 80% of dust is dead skin—if that's totally wrong—then how did the myth get started in the first place? Surely some dust must be dead skin, but how much?

Well, the good strategy is to start broad. How do you define dust exactly? Well, dust is generally defined as particles that can become airborne for a significant period of time when perturbed by natural forces. But how long is a significant period of time, and what are natural forces exactly? So, the definition of dust has fuzzy boundaries, kind of like dust itself.

If you ask the question, "How big is a dust particle?" you get different answers. The International Standardization Organization defines dust as any particle smaller than 75 micrometers in diameter; that’s roughly the width of a human hair. The glossary of atmospheric chemistry terms includes anything up to 100 micrometers. But I've found several papers that include particles as large as two millimeters! This large discrepancy is due to the fact that the most important property of a dust particle is not its size but its settling velocity, which then determines how long it can stay airborne.

A 100 micrometer metal ball would fall to the ground very quickly, while a two millimeter long cotton fiber could float on indoor air currents indefinitely. To get around the different sizes, shapes, and densities of dust particles, the International Standardization Organization has a way of calculating an effective diameter. They define it as the diameter of a hypothetical sphere of density one gram per centimeter cubed, having the same terminal settling velocity in calm air as the particle in question, regardless of its geometric size, shape, and true density. By any of these metrics, a single human skin cell with an average diameter of around 30 micrometers would count as dust.

So then, how many dead skin cells are we shedding? You'd think the average rate of skin shedding would be well established, but the internet is a tangled web of misquotations and missing citations when it comes to skin. In trying to answer this question, I found dozens of web articles saying we shed anywhere between 30,000 skin cells per day to 300,000 skin cells per minute and estimated weights of dead skin as high as nine pounds a year per person. Using Google Scholar, a great resource for searching published scientific research, I was able to track down several peer-reviewed sources that agreed on the numbers.

Every hour, you create about 20 million new skin cells. As those new skin cells form, they push older cells up through the layers of the epidermis, and over a period of weeks, they flatten out and harden, forming the barrier that protects our bodies from the outside world. Ultimately, these dead skin cells fall off, usually one by one, for healthy skin. So, we are constantly molting like a snake. We just don't notice it because our skin comes off one cell at a time.

Fun fact: dead skin cells are shed more rapidly from your forearms than from your back, and more rapidly from your back than from your abdomen. One of the ways scientists have measured these rates of skin shedding is actually by taping containers onto people's skin and collecting up the dead skin cells shed over at least a 48-hour period. Yeah, I'm gonna regret this. Based on these measurements, we know that each square centimeter of your body is shedding around a thousand dead skin cells per hour.

Now, the average adult body has a surface area of nearly two square meters, meaning you are shedding nearly 20 million dead skin cells per hour. That adds up to half a billion dead skin cells per day. Now half a billion dead skin cells weigh between one and two grams; that’s just a little bit less than the weight of a penny. Over a year, that means you shed half a kilogram, or over a pound of dead skin. If you want to think about it another way, in a single year, you give off around 180 billion dead skin cells. That’s roughly the same number as there are stars in our Milky Way galaxy.

An average family of 2.6 people could cover the entire horizontal surface of an average 2,000 square foot home with a layer of skin cells one cell deep in around 200 days. That is assuming all their dead skin cells accumulate around the house, which of course is not true. Some will come off in the shower, and some will get captured in clothes and bed sheets and be rinsed out in the wash, not to mention dead skin cells shed outside the home. Still, the amount of skin we shed is not small.

But how much is it as a percentage of household dust? Well, that turns out to be a trickier question because there are a lot of other sources of dust, like pollen, fibers from rugs, clothes and furniture, dirt from outside, even micro meteorites. Dust from space. To clear things up, I called around to some dust experts, and they pointed me to this book, "House Dust Biology" by Johanna van Brunswick from 1981. Sure enough, on page 37, there is this graph.

Now, it appears to show the fraction of skin particles in airborne dust as 80 percent except not really. This is a stacked area graph, so dead skin accounts for only 20 percent of dust particles between 100 and 300 micrometers. Could a simple misunderstanding of this graph be the source of the urban legend? I mean, it's possible. But what's more interesting to me is that the legend is not that far off. According to this, a full 50 percent of dust particles under 100 micrometers is dead human skin.

The book also reports on a study where they vacuumed a mattress and studied the resulting dust ball under the microscope. 53 percent of the dust was skin particles. So, the debunkers are debunked! While not 70 to 80 percent, dead skin cells do make up a significant portion of household dust. Of course, the exact percentage depends a lot on how much other dust is contributed by the environment. These studies were conducted in houses in the Netherlands, which typically have hardwood floors and therefore less dust than homes with carpeting.

It also depends on how large of a particle you consider dust. Dead skin cells account for roughly half of these small dust particles, but much less than half of what is sucked up by your vacuum cleaner. Oh, that is gross! Unbelievable. I guess it makes sense that the place where we find the most dead skin cells is in and around our beds where we spend a third of our lives shedding a third of our skin in our sleep.

So then, is it true that your mattress actually doubles in weight every 10 years? This claim was actually published in a major news outlet 20 years ago, with the increase in mass attributed to dust mites that feed on your dead skin. The claim, which lacked a scientific reference, is so disgusting it has spread throughout the internet, but it's not true. If two people slept on the same bed for a decade, and even if all their dead skin cells ended up in the mattress, it would only gain around three kilograms, or seven pounds.

By conservation of mass, the dust mites that feed off that dead skin could not weigh more than that, so the total weight gain would have to be less than 10 percent the weight of an average mattress. And remember, dead skin cells are small and light enough to become airborne. Just making the bed has been found to increase the number of skin flakes in a cubic meter of air from 21,000 to 107,000.

Dust is inevitable, and it occurs in higher quantities where there are more people. Scientists have studied the airborne dust in the London Underground and found that fragments of dead skin cells make up around 10 percent of all small dust particles by weight. It’s not just skin; we are shedding a ton of tiny organisms that live on our bodies—a teeming microbiome of bacteria, fungi, and mites.

Every hour, we shed approximately 1 million microbes in a cloud that spreads out a radius of about one meter from our bodies. In one study, an individual was placed in a clean room for 90 minutes. Scientists then identified who had been in the room—not using their DNA, but rather using the characteristic fingerprint of their microbial cloud. The technique was so accurate that the team wrote that it clearly suggests a forensic application for indoor bioaerosols.

I mean, one day we will likely use microbial dust clouds to solve crimes. Dust can reveal a lot about us because a significant fraction of it literally is us. I mean, everyone has heard the expression "dust to dust," but they're probably not thinking about how each and every day, part of us is becoming dust. You know, when I was reading this "House Dust Biology" book, I found some sections that sounded to me a little bit like poetry.

So studying dust from an office under a light microscope revealed tree fibers from paper, eraser dust, rubber, calcite, and pumice, human hair, various dyed wools, cottons, and synthetic fibers such as nylon, rayon, cellulose, triacetate, and Orlon, dander, tobacco, cigarette ashes, graphite, wood shavings, oil, soot, paint chips, glue, fingernail filings, and traces of quartz and starch. I love all these words because they bring vivid images to mind; it's like the tiniest fragments of our lives can paint a full picture of reality—of who was there and what they did.

You know, searching through dust is a little like searching the internet. There are so many pieces; it's fuzzy, nebulous. But with the right tools and a bit of perseverance, I'm confident that we can uncover the truth buried in there. In this case, the truth is that dust, particularly these small particles and dust around beds, is mostly dead skin. Why does that matter? Because it is something we can be certain about.

I mean, for centuries, science has beaten back the shrouds of ignorance, and knowing what is really true is the only way we have been able to make progress. The internet gives us a great opportunity to share that knowledge, but only if someone takes the time to research and establish what really is true. To perform a lot of this research, I have of course been using Google, which might make you wonder how they figure out what to show you in the first place.

Well, good news! Google is not a black box. They have an entire website that takes you under the hood of the search engine and explains all the testing and improvements they make. Check it out by clicking the link in the description. And when you go to make your next search, try it a few different ways: start broad, don't include your answer in the query, use image search, and most importantly, remember: Google shows you the most relevant information—it's up to you to decide what to do with it.

Thanks to Google for sponsoring this video. Learn more about search by clicking the link in the description.