Does the MacBook Air have a Fast User Switching (Login Window) shortcut key?
There isn't one that I'm aware of, but one could use this workaround:


Open Automator, create a new Service.
Set the dropdowns at the top to "Service receives 'no input' in 'any application'"
Add the action "Run Shell Script"
Paste this command (all one line) into the action: /System/Library/CoreServices/Menu\ Extras/User.menu/Contents/Resources/CGSession -suspend
Save the service. You can call it whatever you like.
You can confirm that it's there by going to any application menu, going to services, and verifying that it shows up.
Head over to System Preferences, and into the Keyboard pane.
In the "Keyboard Shortcuts" tabs, select "Application Shortcuts" from the list on the left.
A list of shortcuts should load on the right, with "All Applications" at the top. Highlight this, and click the plus (+) under the list.
In the window that appears, make sure that "All Applications" is selected in the top dropdown.
For "Menu Title", enter the name of the service you created exactly as it appears - including any punctuation, capitalization, etc.
Assign any shortcut you like, then click "Add".


This will work even if the Fast User Switching menu extra isn't enabled.