Someone once suggested that the reason that BD, which is made in the US, is actually "imported" from Italy to the US is that it has a much higher level of lead, and has been outlawed in the US in many applications like house paint. So the theory goes that they quit selling it in the US that's also why Master chalk went south a while back when they reduced/removed the lead and made the chalk not so sticky and smooth as the BD. I found this on the "Billiards, its history and influence" web site:
The Arizona Department of Health Services has warned that at least two brands of green pool-cue chalk available in Arizona Ä Master and Pioneer Ä have been found to contain dangerously high levels of lead. Although infants and children are at greatest risk, chronic lead exposure can cause swelling of brain tissue, fatigue, headaches, irritability and kidney effects. Large exposures can even result in death. The ADHS, in coordination with Samaritan Regional Poison Center, tested 18 brands of billiard chalk at a laboratory in Golden, Colo. after pool cue chalk was found to have played a role in the lead poisoning of a two-year-old Phoenix boy. Master green chalk, made in Chicago, tested as high as 8,000 parts per million of lead, and Pioneer green chalk, made in Taipei, Taiwan, exceeded 7,000 ppm. By comparison, lead-based paint, which was discontinued for use in housing in 1978, has a standard lead content of 5,000 ppm. Cheryl Carpenter, a certified poison information specialist, said 40 micrograms per deciliter of blood is considered a toxic level of lead for an adult. That is twice the toxic level for children. Carpenter said the danger for adults is not substantial and is not an issue for an adult who plays pool occasionally. "I'd be concerned about (people who manufacture the chalk), or pool hall employees dealing with it constantly," she said. Other brands tested in the Samaritan study were found to contain either no lead or very insignificant traces of lead. Those include Imperial and National Tournament brands of green chalk, Superior, Triangle and National Tournament brands of orange chalk and all brands of blue chalk tested.
If you google "pool cue chalk lead poisoning" you'll find several articles in medical journals detailing cases of poisoning from pool or snooker chalk, and test results as well. Never seen an "official" ruling that says lead is the reason, but knowing how lawsuit averse american manufacturers have to be, it seems to make sense.... just sayin'.........