TWP 100 Series has a V.O.C. of 550 grams/liter. Certain states now require lower V.O.C. limits in wood preservatives (350 gram/liter). TWP 1500 Series is the same base formulation as TWP 100 Series with minor formulation changes to comply with the state regulations. TWP 1500 Series can be used in all 50 states. Both lines are designed for the same uses and perform the same. The decision to use TWP 1500 Series should be based on the need to meet your state regulations. Click here to see the listing of LOW V.O.C. States. There are some colors only available in the TWP 1500 Series. If you choose one of these colors you can order them no matter which state you live in. Please note that most retailers stocking TWP 100 Series do not stock TWP 1500 Series.