How many organisms live in my house?

Organism HouseThis is a tricky one to answer because the total number of uninvited houseguests depends on a few different factors, and involves an awful lot of educated guesswork.

Bacteria are the biggest source of cohabiters. They live almost everywhere around the house – coating every surface imaginable, and each bacterial cell is a living organism, happily going about its business. Warm, moist places harbour the most bacteria, as do places where there is food or faeces (or both).

Let’s start to tally up the bacteria from around the home. As you might expect, the kitchen and the bathroom are hotspots for microscopic metropolises.

A US study found that a kitchen drain can contain somewhere around 50 million bacteria, a bathtub 20 million, a kitchen sponge 1.5 million, and a toilet bowl over 500 million! Adding in some more for the combined inhabitants of less bacteria-infested areas like work surfaces and taps brings us to a total of roughly 700 million bacteria on household objects.

However, a much higher number of guests live in the dust on the floor – up 6700 bacteria per milligram of house dust. An average 3-bedroom house has a floor area of 85m2 and will contain about (time to get out the vacuum), adding up to a total of 6.7 billion bacteria on our floors!

Dust is also home to dust mites – microscopic critters that live off our dead skin cells and can cause allergies and asthma – not the most pleasant of house guests. There are roughly 300 mites per gram of dust and as many as 6 million per bed (sweet dreams), adding another 20 million or so beasts to our total.

But by far the largest population of organisms live inside the human body itself. The cosy warmth of a human body is a perfect home for bacteria, and there’s a reasonable estimate that there are about 100 trillion bacterial cells living in an adult human (that’s about ten times more than we actually have!). Most of these are in the intestines and are generally helpful, aiding tasks like digestion.

Let’s say that a four bedroom house has that average family of 2 adults and 1.8 kids. For simplicity’s sake, we’ll say that 1.8 kids approximates to 1 adult – and that there are no pets. So that makes about 300 trillion bacteria.

Add in the 700 million bacteria from household objects, the 6.7 billion from the floor, plus 20 million dust mites (and 3.8 people) and you get… still about 300 trillion living organisms in a home.

For reference, 300 trillion is roughly three thousand times more than the total number of people who have ever lived. In other words, quite a lot. So just be grateful that you don’t have to cook breakfast for them all each morning.

Answered by Lewis


Kärkkäinen, et al. (2010). Determination of bacterial load in house dust using qPCR, chemical markers and culture. Journal of Environmental Monitoring, 12(3), 759-768. DOI: 10.1039/B917937B

Human Microbiome Project Consortium. (2012). A framework for human microbiome research. Nature, 486(7402), 215-221. DOI: 10.1038/nature11209

Colloff, Matthew J. (2009). Dust Mites, 304

American College of Allergy, Asthma & Immunology (2009),House Dust Allergy

Web MD (2007) Top Spots for Bacteria at Home

Image credit: Kali on flickr

Article by Lewis Pike

June 24, 2014

Lewis studied BioMedical Sciences at Liverpool John Moores University and got a PhD in Microbiology and Immunology at University of York. He now spends more time helping people understand science writing than he should and wishing his colleagues would write more clearly for the public as well as each other.

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