In this sense, creatine is a «true supplement» meaning that you are increasing natural levels of something you are already consuming or have in your body. (gymjunkies.com)
The results summarized predictions of existing sound levels, estimates of natural sound levels, and the amount that anthropogenic noise raises levels above natural levels, which is considered noise pollution. (sciencedaily.com)
The researchers ran earth model simulations in which greenhouse gases were kept at natural levels. (climatecentral.org)