What Is the Hygiene Hypothesis?
The hygiene hypothesis says a child's environment can be "too clean," and the lack of exposure to germs does not give the immune system a chance to develop resistance to diseases.
Credit: Sebastian Kaulitzki, Shutterstock

Many parents believe that their children must be kept in an environment that is as clean as possible, but some research suggests that being exposed to what many would call unclean conditions is good for a child's immune system. Research has indicated that children who are kept in very clean environments have a higher rate of hay fever, asthma and a wide range of other conditions. This is what is called the hygiene hypothesis.

The hygiene hypothesis was first introduced in the late 1980s by David P. Strachan, a professor of epidemiology, in the British Medical Journal. Strachan found that children in larger households had fewer instances of hay fever because they are exposed to germs by older siblings. This finding led to further research that suggests a lack of early childhood exposure to less than pristine conditions can increase the individual's susceptibility to disease.

For example, in the late 1990s, Dr. Erika von Mutius, a health researcher, compared the rates of allergies and asthma in East Germany and West Germany, which unified in 1999. Her initial hypothesis was that East German children, who grew up in dirtier and generally less healthful conditions, would have more allergies and suffer more from asthma than their Western counterparts. However, her research found the opposite: children in the polluted areas of East Germany had lower allergic reactions and fewer cases of asthma than children in West Germany. 

Further research has found that children in developing areas of the world are less likely to develop allergies and asthma compared with children in the developed world. 

The idea is simple. When babies are inside the womb they have a very weak immune system because they are given protection by their mother's antibodies. When they exit the womb, though, the immune system must start working for itself. For the immune system to work properly, it is thought that the child must be exposed to germs so that it has a chance to strengthen, according to the U.S. Food and Drug Administration (FDA). 

The idea is similar to the training of a body builder. For a body builder to be able to lift heavy objects, the muscles must be trained by lifting heavier and heavier objects. If the body builder never trains, then he will be unable to lift a heavy object when asked. The same is thought to be true for the immune system. In able to fight off infection, the immune system must train by fighting off contaminants found in everyday life. Systems that aren't exposed to contaminants have trouble with the heavy lifting of fighting off infections.

Mutius hypothesized that the reason children who are not exposed to germs and bacteria are sicklier is due to how the human immune system evolved. She thinks there are two types of biological defenses. If one of the defense systems isn't trained or practiced enough to fight off illness, the other system overcompensates and creates an allergic reaction to harmless substances like pollen.

Research by other scientists has found similar results. Exposure to germs triggered an internal inflammatory response in children who were raised in cleaner environments, leading to ailments such as asthma, according to a 2002 article in Science magazine.

One researcher has personal experience has leads him to back the hygiene hypothesis. "I believe that there is a role in the development of a child's immunity exposure to various germs and a vast microbiome diversity," said Dr. Niket Sonpal, an assistant professor of clinical medicine at Touro College of Osteopathic Medicine, Harlem Campus. "I was born in India but moved to the U.S. and went to college in Virginia and medical school in Europe. I am sure that the vast change in environment has played a role in my immunity. How has it? I don't think we know just yet." 

In 1997, some began to question if there is a correlation between the hygiene hypothesis and vaccinations. The number of children getting vaccinations was going up, but so were the number of children afflicted with allergies, eczema and other problems. Could depriving the developing immune system of infections using vaccines cause the immune system to eventually attack itself and cause autoimmune diseases like asthma and diabetes? This is a highly contested issue. 

Three studies conducted in the 1990s showed that vaccines had no correlation with children developing allergies and other ailments later in life. In fact, vaccinations may help prevent asthma and other health problems other than the diseases they were intended to prevent, according to The National Center for Immunization Research and Surveillance. The idea that vaccinations can cause health problems does not consider the fact that children, whether vaccinated or not, are still exposed to pathogens that help build the immune system. These pathogens also have no relation to the diseases that the vaccines prevent. 

The conflict between cleanliness and exposure can leave parents feeling confused. There are many microbes that can make children very sick, such as such as respiratory syncytial virus (RSV), E.coli and salmonella. So cleaning the home is still very important. What should children be exposed to and what should they be protected from? 

The CDC recommends regularly cleaning and disinfecting surfaces in the home, especially when surfaces have been contaminated by fecal matter or meat or have come in contact with those who have a virus. Children are also encouraged, though, to play outside, even if they may get dirty in the process. This balancing act may prove to help children stay healthy while still developing a healthy immune system. 

Sonpal thinks that the healthy growth of the immune system isn't just about coming in contact with dirt. It also has to do with what foods are consumed, what kind of environments the person grows up in and intrinsic genetics coupled with physical activity levels. Harvard Medical School noted that getting plenty of sleep, avoiding cigarette smoke, drinking in moderation and controlling blood pressure also all play a part in a healthy immune system.

Additional Resources