Porosity affects curl type by determining how easily hair absorbs products. Nonporous hair, which floats in water, requires heavier creams to absorb the product, while porous hair, which sinks in water, requires lighter, water-based products. Determining porosity can be done by simply dunking hair in water. Porosity is hereditary or caused by damage and can be affected by factors such as heat, over-processing, and natural weather. Understanding porosity levels can help in choosing the right products for curly hair.