No, we do not “know” this.
““If you are around somebody who is using e-cigarettes, you are breathing an aerosol of exhaled nicotine, ultra-fine particles, volatile organic compounds, and other toxins.” Dr. Stanton Glantz, Director for the Center for Tobacco Control Research and Education at the University of California, San Francisco…
Electronic smoking devices (ESDs) do not just emit “harmless water vapor.” Secondhand aerosol (incorrectly called vapor by the industry) from ESDs contains nicotine, ultrafine particles and low levels of toxins that are known to cause cancer.
ESD aerosol is made up of a high concentration of ultrafine particles, and the particle concentration is higher than in conventional tobacco cigarette smoke.
Exposure to fine and ultrafine particles may exacerbate respiratory ailments like asthma, and constrict arteries which could trigger a heart attack.2
ESD aerosol particles are smaller than 1000 nanometers, which is a similar size to tobacco smoke and diesel engine smoke, and bystanders can be exposed to this aerosol. “The exact size distribution depends on the chemical composition of the electronic cigarette liquid, the e-cigarette device operation, and user vaping preferences.” At least 10 chemicals identified in ESD aerosol are on California’s Proposition 65 list of carcinogens and reproductive toxins, also known as the Safe Drinking Water and Toxic Enforcement Act of 1986. The compounds that have already been identified in mainstream (MS) or secondhand (SS) ESD aerosol include: Acetaldehyde (MS), Benzene (SS), Cadmium (MS), Formaldehyde (MS,SS), Isoprene (SS), Lead (MS), Nickel (MS), Nicotine (MS, SS), NNitrosonornicotine (MS, SS), Toluene (MS, SS).4,” (source: nosmoke.org)
Obviously the dose makes the poison and it is unknown if long-term exposure to these substances in the concentrations present in secondhand vapor pose significant harm to users or non-users. But we do not “know” it is safe.