This question cannot easily be answered as there are several variables involved that would affect the determination. Ventilation, proper air/fuel ratio at the burner, sufficient oxygen supplied to the burner and the size of the burner are all major factors to consider. The best advice would be to ventilate as best you can and get a good quality carbon monoxide detector with a digital readout. No need to over do it though and your windows are likely more than sufficient. Natural gas is really no different from propane as far as carbon monoxide is concerned. It's not the gas itself that makes a difference, it's the air/fuel mixture that really counts. That and proper ventilation. Be careful though, the consequence of not addressing these issues can be fatal.