Cases of childhood autism are up sharply, and there’s reason to wonder whether the number of cases is much higher than we suspect.

The Centers for Disease Control and Prevention reported in March that autism cases in the United States had increased 78 percent since 2002. That’s 1 in 88 U.S. children (1 in 54 boys and one in 252 girls), or about 1.1 percent of children.

A few weeks later, in May, a new study from South Korea reported that 2.3 percent of children there have autism. That’s twice the prevalence that the CDC has reported in the United States. Why such a difference?

The Korean study, lead by Yale and George Washington universities, counted cases differently. Researchers rigorously assessed individual children ages 7 to 12 in a community of 488,000 to identify known cases. In contrast, the CDC relied upon records of existing cases kept by health care and special education agencies.

Regardless of measuring methodologies, autism spectrum disorder now affects more children than diabetes, AIDS, cancer, cerebral palsy, cystic fibrosis, muscular dystrophy or Down syndrome – combined.

Why is autism on the rise? Better diagnosis alone can’t account for it.

The most cogent scientific explanation is also my greatest concern: chemical exposures. Chemically susceptible mothers who do not know they are susceptible and who therefore do not avoid exposures are at particular risk. They are more likely to have babies who share their susceptibility genes, and be raised in homes and environments in which exposures are relatively high compared to earlier generations. It’s a case where genetics loads the gun and the environment pulls the trigger.

Women who know they are chemically intolerant will work hard to help their families avoid things like pesticides, solvents, combustion products and other exposures that could lead to neurodevelopmental difficulties. This is because they are personally more aware of the adverse effects of these exposures. They will want to protect their children. However, women who are highly “masked” are less likely to avoid exposures. Although they may have multiple intolerances to foods, everyday chemicals and medications, they may be unable to tell which exposures are causing symptoms because triggering exposures occur throughout the day and their symptoms overlap in time.

The use of petroleum-based chemicals has risen dramatically in recent decades. These substances can seriously affect the health of susceptible people. With intense or long-term chemical exposures, these individuals can acquire Toxicant-induced Loss of Tolerance, or TILT. The problem is, currently, it is not always possible to know beforehand who is susceptible—before the parents build a new home or remodel their existing home for a new baby, hire an exterminator to keep bugs away and install air fresheners throughout the house so the home smells nice for baby.

New research suggests the children of chemically intolerant individuals inherit some of their parents’ same susceptibility to developing life-changing chemical, drug and food intolerances if sufficient exposure occurs (e.g., to pesticides, chemicals associated with new construction). The person develops TILT, or as some affected persons refer to the process, they become “TILTed.”

Most but not all chemical exposures are easy to identify by odor. Complex mixtures of chemicals emitted from everyday products used for home construction and furnishings, for example volatile organic chemicals outgassing from new carpet, adhesives, fragrances, etc., can be trapped inside our energy efficient, tightly constructed homes, schools and workplaces. These exposures can be hard to avoid and because of “olfactory adaptation,” our ability to smell complex low-level mixtures is quickly lost—within minutes of entering a home. Think of homes you may have visited while touring builders’ open houses: in your first few breaths you can smell the air, but after several breaths your nose adapts and you are no longer aware. A useful exercise can be to spend several days away from a suspect environment and then return, noting any odors that are apparent within the first breath or two. Or ask neighbors or friends what your house smells like — e.g ., mothballs, new carpet, a fragrance, natural gas or a mixture of substances?

Many of the same environmental exposures that initiate TILT can also interfere with neurodevelopment in a fetus. The harm can start as early as the first month of pregnancy. That’s the stage when the neural tube forms but before most mothers know they are is pregnant.

There are important relationships between autism, genes and exposures. Humans have different thresholds for becoming chemically susceptible. These differences are normal and not defects. These differences are not new. What is new? Our exposures.

Since World War II, the petrochemical era has ushered in countless new chemical exposures. Many of these have found their way into our building interiors where Americans spend on average 90 percent of their day. A mother and infant may spend even more time at home. Not only do indoor exposures vary from house to house and workplace to workplace, but there can be as much as a 10,000-fold difference between individuals in our ability to detoxify and eliminate certain substances from our bodies.

The rise in autism spectrum disorders seems to follow the rise in petrochemical usage, and arouses enough suspicion to justify immediate research.

The need for research is great, and future studies need to focus on potential environmental causes and especially the indoor environment—an area that falls between the cracks in terms of government research funding. There is a saying: “There are no genetic epidemics.” If a condition has become as prevalent as autism, then research must examine potential environmental contributors — and quickly. Although genes play a role in terms of individual susceptibility to exposures, it is the exposures that are the problem, not our genes! In March, Mark Roithmayr, president of the influential organization Autism Speaks, which helped fund the South Korean study, called for a comprehensive national strategy to address the autism epidemic. Among other things, he cited a need to:

• Fund more basic science uncovering the genetic underpinnings of autism.
• Fund more environmental research detecting the causes of autism.
• Accelerate the funding and development of effective medicines and treatments.

I agree with his statement. There are many different types of autism. Autism is an umbrella diagnosis, one that has many potential root causes. However, the “chemical connection” seems be one of the most promising paths toward an understanding of autism spectrum disorder.