Fact Check: You Could Have More 'Life Forms' on Your Skin Than There Are People on Earth

Laerke Christensen

7 December 2024 at 10:00 pm·5-min read

Claim:

There are more "life forms" on a person's skin than there are people on Earth.

Rating:

What's True:

Earth's population is estimated to be roughly 8 billion people, according to the United Nations. Depending on body size and thus the surface area of skin, a person could have more than 8 billion individual microbes living on their skin, according to a 2015 peer-reviewed study and the U.S. National Library of Medicine.

What's False:

However, smaller people with skin that covers smaller surface areas might have fewer than 8 billion individual microbes on their bodies. Also, a "life form" could be defined as a species of microbe or an individual organism. If using the former, scientists estimate a person has about 1,000 on their skin.

Posts claiming that a person carries more "life forms" on their skin than there are people on Earth have long circulated on social media, dating back more than a decade.

For instance, one Reddit post claimed, "There are more lifeforms living on your skin than there are people on the planet."

(Reddit user u/butrcupps)

The claim's level of factualness depends on a person's size and how "life form" is defined. The Earth's population is estimated to be roughly 8 billion people, per the United Nations. Depending on body size and thus the surface area of skin, a person could have up to 20 billion individual microbes living on their skin, according to a 2015 peer-reviewed study and figures from the U.S. National Library of Medicine.

However, smaller people with skin that covers smaller surface areas might have fewer than 8 billion individual microbes on their body.

Also, a "life form" could be defined as a species of microbe, such as bacteria, or an individual organism such as Staphylococcus aureus, which causes skin infections. If using the former classification, scientists estimate that a person has about 1,000 species of microbes on their skin.

Answer Depends on Body Size, the Definition of 'Life Forms'

A 2015 peer-reviewed study by microbiologist Anthony M. Cundell stated that there are up to 1 million microbes living on each square centimeter of skin.

The normal microbial counts using culture methods typically range from 10^3 [1,000] to 10^4 [10,000] organisms per square centimeter with counts reaching a high of 10^6 [1 million] per square centimeter in the most humid areas such as the groin and axilla. (Cundell, 2015)

According to the U.S. National Library of Medicine, depending on body height and weight, a person's skin covers an area of up to 2 square meters:

Even at its thickest point, our skin is only a few millimeters it is still our heaviest and largest organ, making up about one seventh of our body weight: Depending on your height and body mass, it weighs between 3.5 and 10 kilograms (7.5 and 22 pounds) and has a surface area of 1.5 to 2 square meters.

Using these figures, we calculated a person could have as many as 20 billion individual microbes living on their skin, rendering the claim true. Or, someone could have as few as 15 million, rendering it false. The total would depend on a person's height and size, as well as the microbes' density of the microbes.

However, the math works differently if "life forms" are interpreted to mean species of microbes.

The U.S. National Institutes of Health said in 2022 that around 1,000 species of microbes live on a person's skin. In other words, under this interpretation, fewer species of microbes live on a person's skin than there are people on the Earth.

Scientists have long studied the amount of microscopic organisms not just living on humans' skin but also living inside their bodies.

For example, research published in 1977 estimated the ratio of bacteria to human cells in the body was 10 to 1. That finding was based on figures in a separate paper, published in 1972, that estimated the human gut contains 10^14 — 100 trillion — individual microbes.

The New York Times has reported on 2016 research challenging those older studies. In January 2016, a group of Israeli scientists published research drastically reducing the estimated number of individual microbes inside a person's body to 39 trillion.

While that amount far exceeds the population of Earth, the figure is an estimate for the whole human body, not just skin.

Sources:

"Compound Produced by Bacteria Protects the Skin." National Institutes of Health (NIH), 28 Feb. 2022, https://www.nih.gov/news-events/nih-research-matters/compound-produced-bacteria-protects-skin.

Cundell, Anthony M. "Microbial Ecology of the Human Skin." Microbial Ecology, vol. 76, no. 1, July 2018, pp. 113–20. DOI.org (Crossref), https://doi.org/10.1007/s00248-016-0789-6.

"In Brief: How Does Skin Work?" InformedHealth.Org [Internet], Institute for Quality and Efficiency in Health Care (IQWiG), 2022. www.ncbi.nlm.nih.gov, https://www.ncbi.nlm.nih.gov/books/NBK279255/.

Nations, United. "Population." United Nations, https://www.un.org/en/global-issues/population. Accessed 4 Dec. 2024.

Savage, D. C. "MICROBIAL ECOLOGY OF THE GASTROINTESTINAL TRACT." Annual Review of Microbiology, vol. 31, no. 1, Oct. 1977, pp. 107–33. DOI.org (Crossref), https://doi.org/10.1146/annurev.mi.31.100177.000543.

Sender, Ron, et al. Revised Estimates for the Number of Human and Bacteria Cells in the Body. bioRxiv, 6 Jan. 2016. bioRxiv, https://doi.org/10.1101/036103.

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Answer Depends on Body Size, the Definition of 'Life Forms'

Sources:

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