Static appearing on your films is caused by low relative humidity. This causes a buildup and discharge of static electricity when a film is moved across an object such as a screen/cassette, loading bench, or other film. It appears as a plus density (black) defect such as tree branch/lightning patterns and/or a series of black spots. See below for an example.

You can help prevent static with the following precautions:

1. Ground the film loading bench.
2. Install a rubberized mat on the darkroom floor.
3. Install a non ionizing humidifier. Maintain humidity of 30-50%.
4. Handle film carefully when loading and unloading or removing from the film box.
5. Clean the screens with anti-static cleaner and a lint free cloth frequently. Allow the screens to dry thoroughly before use.
6. Wear rubber soled shoes and clothing that does not create static electricity.