Rainwater is usually safe to use as long as it has been collected in a clean container that has not been used before or has not been exposed to any chemicals or other contaminants that could be harmful if ingested or absorbed through the skin. The longer it sits, however, the more likely it is that bacteria from the air will settle inside of the container and start growing on its surface which could make anyone who drinks from it sick with stomach flu or another illness within 24 hours after drinking from it.