A new federal law banning phthalates in children's toys and other kids' products has come into force in the US. But critics are skeptical of the efficacy of the law as companies currently aren't required to publicly disclose the chemicals they use in place of phthalates — and little is known about the health effects of one of the most widely used alternatives.

Phthalates have long been used to make plastic soft and flexible. But the chemicals can leach out of plastic products and enter our bodies, where some scientists suspect they act like hormones. Phthalates have been shown to affect the development of the male reproductive system in lab animals, and a few small studies suggest they may be linked to subtle effects in the reproductive organs of infant boys.

In preparation for the federal ban, manufacturers have been reformulating baby rattles and plastic toys with phthalate substitutes. But the ban doesn't spell out what chemicals they can use to replace phthalates, Sarah Varney reported for NPR, a well-known alternative radio channel.

California enacted a similar ban on phthalates on Jan. 1. That law does require manufacturers to use substitutes that aren't known to cause cancer or reproductive harm — but it doesn't bar substitutes that might have other adverse health effects.

"There are 80,000 chemicals in commerce," says California Environmental Protection Agency toxicologist Stephen Dizio. "We know something about toxicity of about 400 of them. That really means that things come and go in the marketplace that you have no idea what will happen."