I assume you mean
  export_url_vars
rather than
  ad_export_url_vars
(unless ad_export_url_vars was added in 5.0)
If you do, you are still missing the point: The '&' characters in the url's MUST be quoted for the html to be valid. Since most browsers don't care, the problem is not big though.
Here is an example that might cause problems:
<a href="calculate-resistance?volt=2&=3">test</a>