The nation's health experts are unable to gauge the effect of many potentially toxic chemicals on humans because the federal government has failed to study such exposure and has a long way to go" before remedying the situation, according to a report released Tuesday. The study by the General Accounting Office was begun nearly two years ago. The study reviewed more than 1,400 chemicals that pose potential threats to human health and found that only 6% are being tracked by HHS and the EPA. And only a small percentage of the chemicals known or thought to be carcinogenic are being tracked by the government. In some situations where medical experts wanted to collect "human exposure" data - from blood, hair or urine, for example - and examine it for chemicals, they were constrained by financial resources. Such situations included suspected "cancer clusters" or contact with toxic chemicals. State and federal environmental health officials said that current budgets allow them to collect or use such data in less than half the cases where they thought it to be necessary. Even when laboratories have the capacity to collect the data, no laboratory method has been developed for assessing exposure levels in human tissue for many of the 1,400 chemicals known to pose a threat to human health. Data on how environmental toxins affect children are particularly lacking, according to physicians and public health officials who testified at Tuesday's hearing