The lack of certain environment-specific additives gives interior paints a disadvantage when used on external surfaces, and the difference between interior and exterior-formulated paints doesn't end there. Differences begin to emerge when you look at paint's other three basic components, too: pigment, binders and liquids.
Let's start with pigment, which is what provides a can of paint with its color. Interior paints may contain organic pigments for color, and certain types of organic pigments are known to significantly fade if used on an exterior surface; exterior paint formulas avoid these pigments, increasing the time needed between outdoor paint projects.
Similarly, paints are formulated with additives known as binders, which are used to bind the pigment together as well as provide adhesion to the painted surface. Because exterior paints need more help combatting the adverse effects of the environment, binders used in outdoor-friendly formulations give the paint resistance to integrity problems such as cracking and chalking, as well as increased moisture control. Interior paint binders may offer better resistance from abrasion, instead, which isn't necessarily a much-needed quality in exterior paint.
Interior and exterior paints also differ when it comes to the types of liquids used. According to federal guidelines, interior paints -- specifically indoor water-based paints including latex -- are designed to contain the lowest levels of VOCs (volatile organic compounds). VOCs are used as solvents in the liquid component of paint and they vaporize at room temperature. VOCs are linked to both short-term and long-term health problems, from headaches and dizziness (short-term) to respiratory disease and liver damage (long-term). They may also be associated with certain cancers. The EPA reports levels of VOCs may average as much as two to five times higher inside your home compared to outside, and the levels can increase significantly post-painting. Look for interior paints with low-to-no VOC levels. Because exterior paints don't all meet the same low-to-no levels of VOC requirements as paints formulated for interior use (especially oil-based paints), in this instance it's important to remember never to use exterior paint inside your home [source: Environmental Protection Agency].
And for those who just don't know why this has to be so difficult, there is another option; paint formulated to be used both indoors or out. There is such a thing as a hybrid interior/exterior-formulated paint.