FRANKFURT, Germany-Carbon black has been reclassified by a World Health Organization committee as a possible carcinogen after a study questioned whether humans have more in common with rats or mice. The committee of WHO's International Agency for Research on Cancer based the reclassification almost entirely on results of studies in which rats developed lung tumors in an environment of ``gross lung overload with carbon black,'' according to Dr. R.A.F. Cox, a consultant occupational physician in the United Kingdom.
Cox spoke earlier this year at ``Carbon Black World '96,'' which was held in Nice, France.
The rats in these studies were exposed daily to carbon blacks at periods and levels that do not correspond with actual practice, according to German carbon black producer Degussa A.G. The black maker has entered the public debate on this issue by releasing the results of 25 years of medical check-ups on 677 employees at its Kalscheueren, Germany, plant.
Degussa observed ``no increased incidences of skin or respiratory diseases ... compared with the rest of the population,'' said Dr. Rolf Bretistadt, head of Degussa's health service.
He rebutted the position that carbon black should be considered a suspected carcinogen.
The rats in the studies used by IARC developed tumors. But these tumors ``were not invasive, metastasizing or life threatening, and were seen only in male rats not female, nor in mice or hamsters. The rat model may well, therefore, not be relevant to humans,'' Cox said.
The IARC published its first review of carbon black in 1984 when it classified the material Group 3-``not classifiable as to its carcinogenicity to humans.''
Last year the agency called for a review of this classification based on the results of studies performed since the first monograph in 1984.
Following a meeting at IARC offices in Lyon, France, early last year, the committee reclassified carbon black Group 2B as ``possibly carcinogenic to humans.''
``At first sight this seems to be a surprising conclusion as little or no new evidence has emerged in the interim which incriminates carbon black as a human carcinogen,'' said Cox, who was a member of the committee in 1984.
In making their decision, committee members, he said, apparently chose to disregard the findings of two other studies that did not support the change in classification.
Also, epidemiological studies by U.S. carbon black producers and pulmonary function check-ups on 2,500 carbon black plant workers in Europe have found no relevant health risks attributable to carbon blacks, Degussa said.
These results are particularly significant, Degussa said, because they take into account higher levels of black exposure more common in the past.
Degussa is in a ``ticklish'' position, since Germany's advisory committee for dangerous substances is studying the carcinogenic potential of inorganic, non-fibrous, insoluble dust-such as carbon black. The group may issue a ruling on carbon black reflecting the more stringent IARC classification.
Germany's commission on these matters generally is regarded highly, Cox said. Therefore, any change in its position could influence European Union legislation. The current ``indicative limit value'' for carbon black is 3.5 mg/m.
The IARC's findings are not without controversy, Cox said. At the heart of the discussion in the next few years will be the weighting of the ``lung overload phenomenon.''
Whether the ``nuisance dust'' inhalation studies focus on carbon black, talc, titanium dioxide, vulcanic ash or other particulates, they all tend to utilize overload conditions to the point where the dust causes an increase in lung weight, retention of particles in the lungs and marked chronic inflammatory changes, Cox said.
``Such overload conditions never occur in human exposure, even under the worst conditions,'' he said.
Additionally, the tumors in rats do not form in the bronchi, are not transplantable, and are more like proliferative keratin cysts than tumors, Cox said.
``They certainly have no counterpart in human beings,'' he said.
The IARC classifications are recommendations only, Degussa emphasized, and are not legally binding within the European Union.
Degussa said it is informing its customers about the change and is updating its safety data files to pass along to consumers. Degussa said it will ``continue to act on the basis that carbon blacks do not cause cancer in human beings.''