What lies without
Life on the human body
Also in this week's column:
What lies without: Life on the human body
The skin of the human body is alive with life - microscopic life of all kinds. In his classic work, Life on Man (1969), Theodor Rosebury estimates that there are 10m individual bacteria living on the average square centimeter of human skin (155,000 per sq inch).
Rosebury describes all of these robust, active, fertile microscopic creatures as like a "teeming human population during Christmas shopping". The population of bacteria on the 2 sq meters (21.52 sq feet) of the skin surface of the human body varies depending upon what part of the body you examine.
The most bacteria-prone parts of the body are the armpits, the anal region, the pubic region, and the oily sides of the nose. For example, the armpit is the home of up to about 203,000 bacteria per square cm (516,000 per sq inch). Each square cm of human skin consists of about four million cells (10m per sq inch), 24 hairs (60 per sq inch), 35 oil glands (90 per sq inch), 6.1 meters of blood vessels (20 feet per sq inch), 246 sweat glands (625 per sq inch), 7,480 sensory cells (19,000 per sq inch), 23,622 pigment cells (60,000 per sq inch), more than 393 nerve endings (1,000 per sq inch), and all of that microscopic life.
Although all the microscopic life is high in numbers, it is small in size and weight. Rosebury estimates that all of the bacterial life on the human skin surface would fit into a medium-sized pea and possibly weigh about as much.
It is not just bacteria that live on the human skin. We can also become infested with a variety of creatures that set up household on our skin and dine there to their little heart's content.
According to Dr Jonathan Kantor of the Department of Dermatology at the University of Pennsylvania Medical Centre in Philadelphia, there are three types of louse (Pediculus humanus corporis) that can live on humans. They are head lice (Pediculus capitis), body lice (Pediculus humanus), and pubic lice (Pthirus pubis). No prizes for figuring out where on the body each type lives.
There are also the follicle mite (Demodex folliculorum) that lives on the eyebrows, the scabies mite (Sarcoptes scabiei) that lives everywhere, and several other Trombiculae mites (chiggers, redbugs, rougets, harvest, and scrub) that camp out everywhere. In addition there are the tropical rat mite (Ornithonyssus bacoti), the human bot (Dermatobia hominis), the primary screwworm (Cochliomyia hominivorax), and of course ticks, fleas, bed bugs, and a few others. Our skin is a veritable United Nations of tiny critters.
Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to firstname.lastname@example.org
Bad numbers, bad math
The arithmetic in this article is terrible. First, it starts by saying there are 10m bacteria per cm2 (155,000 per in2). Note the ratio, 10m in a small area vs 155,000 in a larger area.
The rest of the article appears to use a ratio of 2.54 cm2 per in2. This ratio is correct for converting between cm and in, but not the squares - for that, you must square the conversion number, reaching something around 6.45.
So if there are 10m bacteria per cm2, then there are 64.5m bacteria per in2. However, this is inconsistent with the rest of the article, wherein the number 203,000 is given as higher than average, the concentration in an average armpit. (203,000 per cm2 is not 516,000 per in2, as stated, it's around 1,310,000 per in2.)