As a general rule of thumb: if you can't keep it clean or it's too severe to clean up, toss it. Sediment buildup This is bound to happen, as it comes from the chemical additives that are put in your tap water to keep it safe and healthy to drink. Calcium is the most prominent cause of shower head buildups.
If you choose a shower head that includes a plastic material, make sure the type of plastic used is ABS. ABS is a non-toxic plastic that is a very strong and resistant to chemicals or corrosion. Thus, making this material perfect for a shower head. ABS is also BPA-free and very heat-resistant.