For an excellent zero VOC (volatile organic compound) bathtub caulk try Eco-Bond LD 50 made for Trim, Tub, Sinks, and Tile.
You can buy it directly from the company's website www.ecobondstore.comand it is even on sale right now for 35% off the regular retail price.Here is the company's product description:
"ECO-BOND LD50 (Light Duty Interior) is a premium waterproof adhesive and sealant that provides a dynamic green friendly replacement to silicones and caulk. ECO-BOND LD50 will provide a lasting seal that remains flexible and sticks to the most challenging surfaces to permanently seal out water. It also offers mold and mildew protection to keep mold and mildew from growing on its seal."
With this product you will not have to worry about offgassing of toxic chemicals and you can apply it safely without having to deal with the noxious smell that you get from traditional products.
They also make zero VOC adhesives for many other purposes for any of your future project needs.
For more information:
Architect Matthew Bremer recommendsLaticrete grout and caulkin his Q&A "We're remodeling our bathroom due to faulty construction. Can you offer some green pointers?"
And check GreenHomeGuide's "Making Your Bathroom Healthy, Efficient, and Comfortable" for green bathroomdesign, construction, and maintenance advice.