Below we reporduce an article by George Glasser and Andreas Schuld of the Children's Environmental Health Network (USA):

While it focuses on the USA it is highly relevant to the UK.

Summary

Young children often stay in the bath from forty-five minutes to two hours.

The exposure to waterborne contaminants in tap water can range from chlorides to a multitude of chemicals.

Depending on whether a child has eaten or if there is residual food in the stomach, about 20-50% of orally ingested contaminants are metabolized.

However, with dermal exposure (ie absorption through the skin), and inhalation, the contaminants are absorbed directly into the blood stream. Virtually 100% of the contaminants are absorbed into the system.

While most of the adverse effects from chemicals are well documented, i.e., occupational exposures and/or oral exposures geared to adults, children's dermal, inhalation or oral exposures are not considered.

Even when dermal exposures are taken into account, the fact that sodium lauryl sulfate (SLS) found in most shampoos, soaps and bubble baths is not.

Drug companies use SLS in medicines to enhance the absorption of medicines internally and through the skin.

Meanwhile there are growing concerns that medical research into the effect of chemicals/medicines on children is based on the false premise that children are merely small adults.

Most toxicological data are based on occupational exposures for adults. However it's clear that a child's system is very different to that of an adult.

Children have a greater surface area to body weight ratio than adults which is thought to lead to increased dermal absorption.

Children's tissues, organs, and biological systems are still developing, with several stages of rapid growth and development occurring from infancy to adolescence.

This rapid development and immaturity of body organs and systems predisposes children to potentially more severe consequences within certain age ranges and windows of vulnerability.

Another factor that can influence a child's vulnerability is that circulatory flow rates are generally higher in children, which may increase a child's susceptibility to toxic effects.

If you are concerned about the effect of bath water on your child check out whole of house water filters.

Full article

YOUR CHILD'S VULNERABILITY TO TOXIC SUBSTANCES IN THE ENVIRONMENT