How are Static Variables shared in a Web Session

How are Static Variables shared in a Web Session

I have an mvc webapi service setup that pulls and pushes data from an sql
server database.
Within the Web project holding the webapi I have a Static class that just
holds some global variables that are accessed from the webapi methods.
Following is a very cutdown example of the static class:
public static class SystemProperties
{
public static int currentContactID;
}
When the WebApi is accessed I strip the ContactID from the Http Headers of
the call and set the SystemProperties.CurrentContactID to it.
Than in the methods of the webapi I access
SystemProperties.CurrentContactID for data calls.
I have found a problem when there is concurrent webapi calls the
currentContactID is getting mixed up.
My question is, how is the static class members shared between calling
sessions? Is it going to be pretty much last in best dressed and if the
previous person is still in there than they will get screwed over by the
new person who will overwrite the variables with their details?
Should I be using a different method to store these details?
Thanks in advance