Children who live in homes with vinyl floors, which can emit chemicals called phthalates, are more likely to have autism, according to research by Swedish and U.S. scientists published Monday.

The study of Swedish children is among the first to find an apparent connection between an environmental chemical and autism.

The scientists were surprised by their finding, calling it “far from conclusive.” Because their research was not designed to focus on autism, they recommend further study of larger numbers of children to see whether the link can be confirmed.

Bernard Weiss, a professor of environmental medicine at University of Rochester and a co-author of the study, said the connection between vinyl flooring and autism “turned up virtually by accident.” He called it “intriguing and baffling at the same time.”

Experts suspect that genetic and environmental factors combine to cause autism, a neurodevelopmental disorder that has increased dramatically in children over the past 20 years.

In the new study, Swedish families were asked questions about flooring as part of research investigating allergies and indoor air pollutants. Phthalates, used to make soft plastic, have in previous studies been connected to allergies and asthma.

The study was based on surveys that asked a variety of questions related to the indoor environment. Of the study’s 4,779 children between the ages of 6 and 8, 72 had autism, including 60 boys.

The researchers found four environmental factors associated with autism: vinyl flooring, the mother’s smoking, family economic problems and condensation on windows, which indicates poor ventilation.   

Infants or toddlers who lived in bedrooms with vinyl, or PVC, floors were twice as likely to have autism five years later, in 2005, than those with wood or linoleum flooring.

“A greater proportion of children with autism spectrum disorder were reported to have PVC as flooring material in the child’s and the parent’s bedroom in 2000 compared to children without autism spectrum disorder,” the scientists wrote in the journal Neurotoxicology. “Furthermore, children with autism spectrum disorder were reported to live in homes with more condensation on the inside of the windows, which may be seen as an indicator for deficient ventilation.”

Children in the study also were twice as likely to have autism if their mothers smoked cigarettes.  The autistic children also were more likely to have asthma.

The lead investigator was Carl-Gustav Bornehag of  Karlstad University in Sweden, who in 2004 found a high rate of asthma and allergies among children living in households with dust containing phthalates.

The scientists reported that they do not know if asthma and autism are related, or whether phthalates contributed to the risk of autism by some other mechanism, such as disruption of hormones. Phthalates in animal tests interfere with male hormones and sexual development.

“The data are far from conclusive. They are puzzling, even baffling, and not readily explicable at this time,” the scientists wrote in their study. “However, because they are among the few clues that have emerged about possible environmental contributions to autistic disorders, we believe that they should be weighed carefully and warrant further study.” Several scientists who did not participate in the study cautioned that it has too many limitations to draw conclusions, but they suggested that new studies be designed to look for a connection between autism and indoor air pollutants.

Dr. Philip Landrigan, a pediatrician who is director of the Children’s Environmental Health Center at Mount Sinai School of Medicine, called the results “intriguing, but in my mind preliminary because they are based on very small numbers.” Landrigan said he has “no doubt that environmental exposures are involved in causation of autism,” but he suspects the most significant exposures occur not in childhood, but early in pregnancy, “when the basic architecture of the brain is still being established.”

The researchers relied on questionnaires and did not measure any chemicals in the homes, which limits the reliability of the findings because they do not know for certain that the children were exposed to phthalates. Previous studies have found that phthalates are common in household dust.

Phthalates are used as softeners in plastic for vinyl flooring as well as other building materials, toys and medical equipment. The chemicals have become increasing controversial in recent years, with Congress last year banning their use in children’s products.

The American Chemistry Council, representing chemical companies producing phthalates, said in a statement Monday that the new study does not prove a link between the chemicals and autism. “No other means for assessing these children existed except for the questionnaire and the parent’s responses, making this finding rather insignificant,” said Chris Bryant, the group’s managing director. Autism, he said, “was not systematically analyzed, but just happened to be a question asked five years into the study.”

The industry group has said flooring emits “extremely low” levels of phthalates. Because the compounds are heavy molecules with low volatility, they do not tend to evaporate, and wear and tear that might release particles into dust is slight, they said.

Vinyl flooring is commonplace in Sweden, where only about 1 percent of homes have carpeted floors. But it is uncommon in U.S. bedrooms, so it may not be related to autism among American children. However, carpeting contains other contaminants, including pesticides and brominated flame retardants, which have been found to harm brain development in animal tests.

The scientists said their new finding “suggests that studies of other chemical contaminants with endocrine disruptor properties might yield useful insights into the genesis of” autism.

Previously, three studies in California have found a connection between children’s exposure to household or agricultural pesticides and autism.

Rates of autism in California have increased seven-fold since 1990, a recent study found. Because genetics do not change that quickly, scientists suspect that chemical pollutants are probably playing a role. But there have been few studies attempting to pinpoint which chemicals, or combination of chemicals.