Polyester is a fabric that is derived from oil, therefore, it is naturally water-repellent. That makes it non-absorbant so it is a great material for active wear which aims to keep water away from the body and skin. This fabric has a moisture regain of only .4% at both standard temperature and in humid environments. Therefore it won't absorb moisture and cause you to be clammy and sticky as a result. Additionally, it offers a moderate to high strength. For all it's many positives, the downside of polyester is it is not biodegradable nor is it considered a green or sustainable fabric.