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Paranormal hobbyists and investigators know about smudging through the ever popular and expanding paranormal media, this includes television shows, books, and radio shows. They use it to cleanse houses of spirits, usually at the end of an investigation after they've done their data collection. Along the line, they were told that smudging removes spirits, and it might, but there is no scientific proof of this.
Smudging is an ancient Native American ritual using natural herbs to cleanse the negative energy from a person or place. There have been scientific studies conducted that prove that burning sage or incense have a positive effect on the human body. The herbs used for smudging act as a natural "purifier" of sorts that cleanse the area of any harmful bacteria. Science shows the effects can last for hours, if not days depending on where a smudging was conducted.
In reality, if investigators choose to use smudging in an effort to cleanse the property or person of any wanted "spirits", the thought should be this may help to cleanse the home, not that it will indefinitely wash the area as there is no scientific proof that it does so. There is nothing wrong with belief, and sometimes that's all you need.