White smoke/vapor in exhaust

My 1990 Ford Econoline Conversion van (75,000 miles) recently started emitting white smoke or vapor from the exhaust pipe. I’ve been told it is steam caused by one of two things - either coolant or automatic transmission fluid leaking into the engine and being burned off. I’ve checked the oil and no sign that coolant has contaminated the oil (no chocolate shake-like substance). My coolant was a little low, so I topped it off. One possibility is this could be a head gasket issue. I have two questions. First, could it be something other than the head gasket? And second, is it safe to run the engine or drive the van while it is emitting the white smoke? Thanks.

There are other possibilities, including simply just condensation in the exhaust pipe.

The white smoke might be the engine burning transmission fluid. On the side of the transmission you’ll see the vacuum modulator with a vacuum hose connected to it. Remove the vacuum hose from the vacuum modulator and if transmission fluid leaks out of the vacuum hose/modulator the modulator is leaking and requires replacement.