If you use a portable propane heater or any non vented propane or gas heater then you should always make sure you have air flow to prevent carbon monoxide poisoning.
It's recommend to crack open a window to get some air flow. Even though that seems counter productive to heating your home.
But a small portable heater will only warm you and not the entire house anyway.
If possible it's best though to get a vented propane or natural gas heater that requires no electricity so you'll be much safer since it's better for natural gas and propane heaters to vent to the outside to prevent carbon monoxide poisoning which can kill you silently in your sleep.