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Environmental Aspects of Autism

Interactive Autism Network at Kennedy Krieger Institute
Date Last Revised: 
June 4, 2013
Date Published: 
October 15, 2007

Experts often say that autism is the result of a combination of genetic and environmental factors.1 If the word "genetic" refers to the blueprint for a person carried within his or her DNA, to what does the word "environment" refer?

In simplest terms, the word "environment" refers to everything outside of us -- the external physical conditions in which we exist. It is the air, water, minerals, plants, animals, chemicals, and all other external factors surrounding and impacting us at any time. We might think of genetic factors as those that affect us from the inside out, and environmental factors as those that affect us from the outside in.

With regard to Autism Spectrum Disorders (ASDs), we might ask: if the rate of ASDs is truly increasing, is there something in the environment that is contributing to this? If we know genetics definitely provides part of the answer, 2  does something in the environment provide the rest? 3,4   We do now know that environmental factors can affect genes in a lasting way without modifying the genetic code itself by, for example, turning genes on or off.  5  The existence of such epigenetic factors means that we cannot always say either genetics or environment caused something to occur. Genetic and environmental factors can interact. This is why scientists can say both genetics and environmental factors may play a part in ASDs.

The same is true if we consider the theory that spontaneous mutations of genes within sperm and egg cells can lead to autism.  6 Genetic material is being altered. In addition to mutations that arise due to aging or other natural processes, there may be one or more environmental factors responsible.

Is there evidence that environmental factors can cause autism? Yes. We know that "outside-in" factors can cause ASDs, because this has already been observed to occur in connection with some medications taken during pregnancy. The major birth defects caused by thalidomide, a drug prescribed in the 1950s and ‘60s to treat morning sickness, included autism. 7  The use of anticonvulsant medications during pregnancy, such as sodium valproate, has also been associated with higher than expected rates of ASDs in children. 8,9,10   Misoprostol, a drug used by women in some countries to induce abortion, often fails, such that children exposed to the drug survive to be born. Many suffer not only from an array of birth defects known as Mobius sequence, but from autism as well. 11,12

Environmental Dangers

No one can deny that our modern world is full of environmental peril, both natural and manmade. Tetanus toxin and black mold are examples of natural substances that can harm human beings. Ultraviolet (UV) radiation is also natural, while the depleted ozone layer that lets more of it reach us is not. Among the list of toxic substances created soley by man are pollution, nuclear waste, food additives, household chemicals, and pesticides.

In the past few decades, countless aspects of our lives have changed rapidly, and all at the same time. We are exposed to a staggering number of toxins (and stresses) never faced by previous generations. Writes one group of researchers investigating the impact of environmental exposures on children’s health:

"The environment that children face today includes new hazards neither known nor imagined a few decades ago. More than 80,000 new synthetic chemical compounds have been developed and disseminated to varying degrees in the biosphere over the past 50 years. Of those 80,000 chemicals, there are approximately 15,000 that are produced in substantial amounts, meaning 10,000 or more pounds per year. Of those 15,000 chemicals, children are at especially high risk of exposure to approximately 3,000 high production volume chemicals that are produced or imported in the United States in quantities at or greater than 1 million tons per year."  13

Might exposure to one or more of these chemicals, or some other environmental danger, be responsible for the increase in ASDs?

Seeking Causes

We still have no single, simple answer to the question: What causes ASDs? There may not even be just one answer, as ASDs vary, and different subtypes might someday be attributed to different causes.

As researchers explore the possibilities, you will hear one scientific principle repeated again and again:

Correlation does not equal causation.

Correlation refers to the strength and direction of a possible relationship between two things. One may increase when the other does, for example, or go down in a consistent way while the other goes up. However, just because two things are associated in such a way doesn’t necessarily mean that one thing caused the other. This is especially true when many things are changing at once.

You could show, for example, that women’s salaries steadily climbed between 1950 and 1980. You could also show that the number of satellites in orbit around the earth increased during the same period. You could not conclude, however, that more satellites were being launched because women were making more money or vice versa.

Nevertheless, figuring out that there is some relationship between two things – that one increased at the same time the other one did, for example -- is at least a first step in figuring out if they might be related. To show that one thing likely caused the other is much more difficult. How do you judge how likely it is that one factor (like a chemical exposure), which is increasing at the same time as a second factor (like autism), caused that second factor? Guidelines include: 14

  1. Timing
    Does the timing of cause and effect make sense? In other words, did the factor that you think might have caused a disease or disorder occur before the disease or disorder did?
     
  2. Strength of the Association
    What is the increase in risk of developing a certain disease after being exposed to various substances? The higher the risk of developing a disease after a person has been exposed to some factor, the more likely that factor is to have caused the disease.
     
  3. Dose-Response Relationship
    As the amount of exposure increases, does the risk of disease also increase? A strong relationship between amount of exposure, or dose, and risk of disease helps to support a claim that this exposure causes the disease.
     
  4. Replication of Findings
    If one factor truly causes a disease, we should find this again and again, in study after study. The more this is true, the more powerful the finding. If four studies try to show the same thing, and no two come up with a similar answer despite studying similar people in similar circumstances, the evidence for causation is weak.
     
  5. Biologic Plausability
    Does the notion that a certain exposure caused a certain condition make sense in light of current scientific knowledge regarding human biology?
     
  6. Consideration of Alternative Explanations for an Association
    Have researchers considered what additional factors may be responsible for an association between a certain exposure and a disease?  Often, when one substance is present, so are others, and these may also be associated with specific socioeconomic or cultural factors.  What seems to be the cause of disease may not be the cause at all.  Say that a researcher finds that living in areas with extremely bad air pollution is associated with higher rates of autism.  That researcher must also consider that areas with bad air pollution might also tend to have polluted groundwater, more lead paint in the houses, or contain more people living below the poverty level who also happen to have less access to prenatal care.   One of these other, related factors -- or one not identified -- could be the real cause of the higher rates of autism.
     
  7. Cessation of Exposure
    If the exposure stops, the risk of disease should go down. If it doesn't, the exposure may not be the factor leading to disease.

Keeping these guidelines in mind will better equip you to evaluate studies trying to establish the cause of any condition, disease, or disorder.

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