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There are two types of kerosene that are used today. They are separated into type 1-K and type 2-K categories, based mainly on the difference in their sulfur content. Type 1-K kerosene is classified as having .04 percent of sulfur by weight as compared to type 2-K which has a higher level of .30 percent sulfur by weight - that's a lot more smelly sulfur. The cleaner type 1-K kerosene is generally used for most kerosene heaters. Type 1-K is refined to a point that is safe to use indoors without negatively affecting humans or household pets.