You should always wear a mask while sanding or painting, whether you are indoors or outdoors. If sanding, use a N95 or N100/P100 class mask. If painting or using a chemical stripper, use a vapor class mask. They're cheap and it's better than getting a chronic respiratory and/or neurological disorder. Trust me on this one.
I contracted Multiple Chemical Sensitivity as the result of unsafe work practices, for three weeks I was configuring a new IP Telephony system for a client who was completely gutting and refurbishing their office. The workspace was not ventilated and all that separated me from the construction was a thin sheet of painter's plastic the workers taped over the door frame. So think about it, 3 weeks X 40 hours = just 120 hours of breathing construction and dry wall dust and that was all it took.
A lot of the 9/11 first responders have the same condition, and all that took was from one day to one week, depending on their level of exposure. Do yourself and your loved ones a favor and please, please, please wear a respirator when sanding or painting, no matter if it's indoors or out. Lead or no lead, it's stuff you don't want in your lungs.
