El Santuario de Chimayo is an old adobe-brick and stucco chapel in the hills of northern New Mexico. It was built in 1816, but a sanctuary has been at this site for much longer. The locals offer many legends about its origins, but the truth is buried beneath the murk of time.

As mystical and beautiful as the sanctuary is and as striking as the crucifix above the altar is, nearly none of the supplicants in these pews today have come to see the sanctuary or the crucifix. Instead, they have come from all over the world to this place in New Mexico to eat the dirt that lies beneath the adobe floor.

According to legend, that dirt is sacred. On some days, the line of pilgrims stretches for blocks. A sign on the wall reads “Please, this earth is blessed. Do not play in it.”

Past the front, beyond the Communion rail, lies a low-ceilinged entrance to a small room. There, a hole (the posito) about 18 inches across pierces the floor of the church. Inside lies the deep-red dirt of Chimayo. That dirt tastes of grit and time and something else — maybe cinnamon or cumin seeds. Maybe salvation.

Pilgrims to Chimayo are not the only humans who eat dirt. Nor are religious reasons the only reasons to imagine that dirt may be special.

Much of who we will become we acquire from the world around us, including — in fact, especially including — dirt. Other than water, what little stuff we humans have inside us is largely dirt. Admittedly, this dirt is sometimes highly processed before we receive it: in the form of cows and sheep and carrots and squash and bison and sorghum.

But not everyone wishes his or her dirt to be so far removed from the stuff of mud pies and mucilage. On every continent (except, possibly, Antarctica), some people intentionally eat dirt, and humans are joined in this practice by myriad rats, mice, mule deer, birds, elephants, African buffalo, cattle, tapirs, pacas and several species of primates.

In the United States, most of us believe humans should eat only food. Because of that, we consider the consumption of nonfood items pathological, even though we know that what people define as “food” varies dramatically both ethnically and regionally. We call eating nonfood stuff “pica.” And since soil does not appear on the Federal Drug Administration’s food pyramid, we define soil pica as a disease.

But there are rules. All dirt eating is not soil pica. It all depends on how much dirt a person eats and whether he or she eats that dirt by choice.

In June 2000, the people at the US. Agency for Toxic Substances and Disease Registry appointed a committee of men and women to review soil pica. The committee settled on pathological levels as consumption of more than 500 milligrams (about 1/10th of a teaspoon) of soil per day. But the committee members themselves conceded that the amount selected was arbitrary.

“Much of who we will become we acquire from the world around us, including—in fact, especially including—dirt.”

In truth, especially when the wind is blowing, most of us consume more than 1/10th of a teaspoon of dirt every day. That would seem to make the declaration of the U.S. Agency for Toxic Substances and Disease Registry worse than arbitrary.

The only human soil consumption considered by some to be even slightly normal is dirt eating by pregnant women (especially in sub-Saharan Africa), migrants from sub-Saharan cultures to other parts of the world (notably the United States), and children worldwide.

For centuries, indigenous peoples have routinely used clays (decomposed rock, silica and aluminum or magnesium salts and absorbed organic materials) in food preparation, to remove toxins, or as condiments and spices. Clays are also often used in medications (for example, kaolin clays in Kaopectate and bentonite clays in antifungals).

But the most common occasion for eating dirt in many societies (the only occasion in some societies) is pregnancy. In fact, in several areas of this world dirt eating is diagnostic of pregnancy. When sperm and egg collide, the world changes. That is obvious. But why pregnant women eat dirt is not.

Eating clay, for several reasons, could be useful for pregnant women. These clay soils contain a lot of kaolin and that could ease gastrointestinal upset common in the first trimester. Also, kaolin clays absorb toxins, some of which might come from plants in these women’s diets, though that wouldn’t explain why they eat dirt only when pregnant. And finally, these clays contain calcium, an essential element for building baby bones.

The most popular of the available dirts are subsurface clays. These soils disappear into pregnant women at the rate of 30 to 50 grams a day (about a quarter cup), although some eat much more. But nutrients are not all dirt eaters are after.

Soil also contains considerable amounts of organic material, including many live microorganisms — especially bacteria and fungi. The human gut is the largest area of direct contact between a person and the world. To function properly, that gut needs to house billions upon billions of bacteria. 

“Eating dirt, rather than being abnormal, may be an evolutionary adaptation acquired over millennia of productive and not-so-productive interactions with bacteria.”

Major portions of human immune systems do their own special voodoo inside human intestines. In fact, these spots in our guts even help to generate some of the white blood cells we couldn’t live without — which makes human gastrointestinal systems mysterious swamps where nearly magical things regularly occur.

Things we eat often immunize as cleanly and as quickly as syringes and needles. And that immunity is regularly passed on to fetuses and babies across the placenta and through breast milk. So maybe eating dirt immunizes moms and then they pass that immunity to their babies, before and after birth. In support of this, it appears monkeys that regularly eat dirt have lower parasite loads and healthier babies.

In some human cultures, people bake the clay before eating it. Curiously, that is the very same thing we do with many vaccines — the so-called heat-killed vaccines. The baking is done to eliminate the microorganisms’ power to reproduce but not their capacity to induce immunity.

Furthermore, for decades we have used aluminum salts — like those found in clays — to enhance the potency of human vaccines, because they stimulate inflammation, and that makes for better immunity.

In pregnant women, this sort of immunity could offer protection against precisely those infectious threats the baby will face at birth. Eating dirt, then, rather than being abnormal, may be an evolutionary adaptation acquired over millennia of productive and not-so-productive interactions with bacteria — an adaptation that enhances fetal immunity and increases calcium, eliminates gastric upset, detoxifies some plant and animal toxins, and perhaps boosts mothers’ immunity. And all of this at times when the hormones of pregnancy, factors produced by the fetus, changes in the serum and cell-surface proteins, and who knows what else suppress the mother’s natural immunologic desire to destroy her fetus — a miracle, nearly. 

My children ate dirt with surprising gusto: garden soil, road soil, leaf-mush soil, sunlit soil, moonlit soil, sod soil, bug-body soil — even gutter soil. As usual with my children, before I could talk them out of this behavior, they gave it up on their own — their behavior depending more on personal likes and dislikes than on my paternal concerns. I was pleased when they quit. Later I was reassured to discover from other parents that their children were just as taken with dirt as mine were, some even more so. I felt less like the parent of a couple of dirt-eating, psychosis-ridden, nutritionally deprived children, even if my children, like me, were never quite “normal.”

Children often begin eating dirt a year or two after birth and may eat soil for some of the same reasons pregnant women and some animals do.

Various maternal antibodies cross the placenta to the fetus before birth. Others appear in breast milk shortly before birth and for a year or more afterward. These antibodies provide much-needed protection in the newborn and last about eighteen months to two years.

But as maternal immunity wanes, eating dirt might “vaccinate” children who are losing their mothers’ immunity. That dirt may stimulate these 2-year-olds to start making their own antibodies. Eating dirt might also help populate intestinal flora — the bacteria in our intestines that are essential for normal gastrointestinal and immune development.

“Children exposed to a little more of the infectious side of this world seem to fare better as adults.”

The Environmental Protection Agency estimates that children in the United States consume, on average, 200–800 mg of dirt per day. Some children regularly consume more than their allotment. Still, while plenty for immunization, that doesn’t seem like a lot of dirt. Nevertheless, we parents have tried for years to put a stop to it. I don’t know of an instance in which anybody has succeeded in keeping children altogether away from dirt.

In some cases, this may be a good thing. It is likely that we mammals are dependent on early and massive infection. Children with many older brothers and sisters are less likely to have asthma, hay fever or eczema. West African children who have had measles are half as likely to have allergies as children who never had measles. Italian students who recovered from infection with hepatitis A had fewer and less severe allergies than fellow students who were never infected. Children with Type 1 diabetes (an autoimmune disease) are less likely than healthy children of the same age to have had infections before their fifth birthdays. Children raised in rural areas, especially on farms, have fewer allergies and autoimmune diseases than children raised in cities.

Apparently, there is a window when infection drives the immune system and the gastrointestinal system toward their proper ends. Children exposed to a little more of the infectious side of this world seem to fare better as adults, to less frequently recognize certain innocuous organisms as a threat, and to more readily discriminate between self and not-self and separate the fatal from the innocuous. 

How dangerous is eating dirt? My mother was pretty certain about this — dangerous. These days, it’s hard to find plain old earth. Most of the stuff we walk on has soaked up many years of human wear and tear. Sometimes that includes PCBs, mercury, lead, and who knows what other by-products of human progress. Because of that, you need to be careful and selective about what soils you sprinkle on your morning cereals or even, depending on where you live, quit eating dirt altogether.

The inherent biologic danger of soil is difficult to assess. Soil unaffected by the pressures of population, industry and agriculture may be vastly different from the soil most of us encounter routinely.

The most common parasitic infection among dirt eaters in the United States is toxocariasis — caused by a roundworm. Many domestic cats and dogs carry this roundworm. A lot of these animals show no or only minor symptoms, but they pass millions of eggs every time they defecate. And the eggs can persist in the soil long after the poop is gone. If a child eats that soil, he or she may develop a serious disease that can cause blindness.

One recent report describes the infection of two children (at separate sites) with raccoon roundworm (Baylisascaris procyonis). This parasite does terrible damage to nerves. Both children suffered severe neurological damage, and one died. The children ate the roundworm when they ate soil contaminated with raccoon poop. So eating dirt can have serious consequences, but it seems that consequences this severe are rare.

In the United States, toxocara infects about 10,000 people every year. As a result, this worm currently infects about 14%t of the U.S. population. Most of those infections occur before age four. Outside of the developed world, toxocariasis is an even bigger problem. So soil contaminated with dog or cat feces provides a major route for movement of parasites from pets into humans. Most of these infections result in few or no symptoms. But some have severe consequences. Every year, nearly 700 people lose their sight in one or both eyes because of a toxocara infection. 

Neither raccoon roundworms nor toxocara are a regular part of soil itself. But clearly knowing what contaminates the soil that you or your children eat is critical. 

Our soil literally teems with living creatures. In spite of that, there is little evidence to suggest that eating or breathing dirt regularly causes disease in people. The few infectious diseases directly tied to eating dirt appear in children who mostly eat topsoils. Certainly, the outermost crust of our Earth is the part most regularly contaminated by what we and our animals do. Because of that, eating these soils may be riskier, especially in certain areas.

Normal, abnormal, and unavoidable, the soil we eat is a large part of who we are and who we will be. The soil eaten by children may even be an essential part of becoming human. 

Here beneath the old wood and marvelous crucifix, I watch the faithful leave. I marvel with them at the miracle beneath this adobe floor, the same miracle buried beneath most every place human feet have trod.

Manufacturing a human being takes a bit of dirt. In truth, we are creatures of starlight and dirt. And though we most often take it wholly for granted, the dirt we walk upon, sit upon, and sometimes toss our trash into is sacred. Please play in it.

Gerald N. Callahan was a professor at Colorado State University with joint appointments in the department of microbiology, immunology and pathology and the department of English. He published poetry, essays and books exploring the processes of being human. Callahan died in 2020. This essay is a modified excerpt from his book “Lousy Sex: Creating Self in an Infectious World.” His last collection of essays will be published in 2023. © Gerald N. Callahan, reprinted with permission of the University Press of Colorado.