Because your web browser must request these advertising banners from the ad network web site, these companies can send their own cookies to your cookie file, just as if you had requested a web page from the site.
Please note that if an advertiser asks the GPWA to show an advertisement to a certain audience (for example, men ages 21-34) and you respond to that ad, the advertiser or ad-server may conclude that you fit the description of the audience they are trying to reach.
If you want to prevent a third-party ad server from sending and reading cookies on your computer, currently you must visit each ad network's web site individually and opt out (if they offer this capability).
Currently, the GPWA has relationships with the following third-party
Atlas DMT, a division of Avenue A, Inc.