A study measuring the aerosols from two vape users and comparing it with secondhand cigarette smoke found vapes produced a much lower level of ultrafine particles, but a much higher level of nanoparticles. It is not clear what risk these pose. Another study found vape use increased fine particles in a room’s air.

From SMH: Is secondhand vaping a growing threat or ‘fake news’?

It would seem the science is still not in on just how dangerous it is, but further in the article it says:

The question, says Demaio, is whether we should wait for conclusive evidence before taking action.

“We don’t have any long-term studies on the safety of these products, or on the safety of secondhand exposure. It took us 20 years to realise it [tobacco] was causing damage in the next generation of kids who were living with others who smoked.

“My worry is we take another two or three decades to realise the same thing.”