An MSDS data sheet is a good starting point but doesn't really give the full story, it just highlights the risks for high levels of exposure to the substance. There are many other things to consider:
For working with glazes in the UK you must either use a lead free or a low solubility lead glaze, where low solubility measures the amount of lead released from mixing the dry glaze in hydrochloric acid. Lead bisilicate releases little lead in this test, so any lead glaze using lead bisilicate can be used.
You must ask yourself how much exposure you have to the material. In the UK when looking at long term effects (as is the case with lead) if you are working with it all the time then this is averaged over an 8 hour period (ie a day), or if you work with it intermittently then this is averaged over a 40 hor period (ie a week). So if you are mixing lead glazes for 1 day a week then the exposure can be 5 times higher on that day than if you do it every day.
Then you need to look at how the toxic material can get into the body. Lead bisilicate isn't absorbed through the skin, and if ingested through the mouth in the quantities likely in a studio using good practice (i.e. no food or drink, no licking of brushes for applying glaze, washing hands/using gloves afterwards etc) is not going to release a significant amount of lead passing through the system, so the main risk is airborne inhalation.
If you look at the HSE airborne limits for lead in the UK, this is 0.15mg/cu.m of lead (corresponding to 0.37mg/cu.m. of lead bisilicate), which is higher than the limits for chromates, nickel, cobalt and vanadium. The question is, how much of the lead in your glaze will get into the air - primarily by using dry materials when mixing the glaze or doing things like scraffito, or by over firing the glaze so it gives off lead fumes in the kiln. And then, how much will be filtered out by your dust mask? This can only be ascertained by measurement, which is beyond the skills or cost effectiveness of studio potters, but guidance from the HSE is in these document:
Also this is a very useful reference on lead glazes not just in manufacturing, but also on tableware:
Yes, lead presents hazards, but if used intelligently they are perfecty manageable, the same as with many other potter's materials. And it is your choice whether to use it or not.