No. It’s normal for your phone to get slightly warm when charging wirelessly, so this is nothing to worry about. All Zens products have passed appropriate safety tests. When using a regular plug-in charger, you’ll notice that the charger warms up a little. It’s the same with wireless charging.
Did you ever doubt the safety of your cooking plate? You can compare wireless charging to inductive cooking, which is basically the same technology. This utilizes much more power (~5000W) than current wireless charging stations (~5-15W). Wireless charging systems must obey a set of rules and regulations related to electromagnetic interference (EMI/EMC), sometimes also called “radio disturbance”, and magnetic fields. All Zens products meet these rules and regulations and are Qi certified. All kinds of precautions have been taken to make sure that wireless charging is safe.