Burning LPG gas creates harmful gases such as nitrogen dioxide, carbon dioxide, and carbon monoxide the effects of which can be serious enough to kill you. How is the public proteceted against these effects? The manufactureres are made to put warning labels on the appliances. Most other countires require the manufacturers to have oxyygen depletion sensors that will cut off the gas supply. Burning LPG also produces about half a litre an hour of moisture which remains within an unvented room causing damp and mould, all of which are likley to exacerbate any person with asthma conditions.
If your a landlord you should make sure that your lease does not allow the use of these heaters as they are a disaster for your asset let alone the health of your tenant.