...and this, from Nails Magazine
In general a person is more likely to develop a reaction to the resin of gels than the powder-and-liquid combination of acrylic. Doug Schoon, vice president of science and technology for Creative Nail Design in Vista, Calif., explains why: “In order for something to cause an allergy, it has to penetrate the skin.” Schoon says that if a molecule is too large, it can’t penetrate the skin. The molecules in the liquid monomer used in acrylic application are at a size where they can penetrate the skin, but when they cure (dry) the molecules increase in size and are unable to enter the body through the skin. The molecules in the ingredients of gel applications are smaller than the molecules in the monomer, which is why gels are more likely to cause a reaction — they make it into the body more easily.