What you need to know:
Many public wifi networks ask for an email address and/or a phone number before access is granted. This identifier allows the wifi provider to identify you and is generally used for their own marketing purposes. However, this data point packaged with location data is also often sold to data brokers to better understand places you visit and your potential interests. This allows data brokers to (for example) categorize an individual as a business traveller if he/she frequently logs into an airport wifi network.
In addition to tracking from companies, public wifi can be susceptible to hacking if proper security measures are not put into place by the person setting up the network. Hackers can position themselves between a connected user and the connection point and receive every piece of information the user sends out to the internet– which can include sensitive emails, usernames and passwords, credit card information, etc. Hackers can also use unsecured wifi connections to distribute malware and conduct phishing attacks.
What to do:
Use a service that creates disposable emails if one is required to access the network.
To stay safe from hackers while using public wifi you can adjust your behavior and settings outlined here.
You can also utilize VPNs which will reroute your traffic through dedicated encrypted servers and will help protect your data.