When a Kerosene heater is burning it produces carbon monoxide that is introduced into the home or space it is in.
That is for the non vented kerosene heaters and if you don't have ventilation you can be killed through carbon monoxide poisoning.
As long as you make sure to vent the house such as leaving a window open a crack to allow fresh air in you should not get sick from the Kerosene heater.
Also make sure to take the fuel tank outside or the Kerosene heater outside to refuel once it has been cooled down and never refuel the kerosene heater indoors as it could cause a fire.
All heaters including Kerosene Heaters can be dangerous if not used properly and safely.
With a Kerosene heater that is not vented to the outside you need to make sure you have a window cracked a little bit to allow some air into the home or space you're heating.
Non vented Kerosene heaters are only meant to be used for emergency temporary heating such as when the electricity goes out and you have no other heat source.
So you need to vent the area they are in and when a Kerosene heater goes out it produces kerosene fumes which can make you sick and they also produce carbon monoxide which can kill you.
So like I said make sure the house or other space has a little ventilation to allow fresh air into the space and you should be fine.
Also make sure to keep flammable items such as curtains, clothing etc 4 to 5 feet or farther away from the Kerosene heater to prevent a fire.
The further away you keep the combustibles the better.
I've used Kerosene heaters for emergency heat for years and never had any health issues or fires because I made sure to keep things away from it and also to make sure the house had ventilation.