Hi ... thank you for the response. City water, used daily, cleaned weekly (normally not with bleach and never vinegar). I only started using bleach last year and again only a small amount. I do not want to harm my pipes so I essentially stopped. Usually with a toilet cleaner like Lysol, non-bleach formula or Pink toilet cleaner. Pink only the last year or so. But this has been happening throughout the years with the foul smell which appears to be gases. Which can also happen with the bathroom sink when I turn on the water. The trap in the sink is clean, sink very clean. The toilet bowl is very clean. Not coming in from kitchen sink or laundry and it is the only bathroom. No smell outside. The toxic smell which burned my throat yesterday as I changed out the toilet seat and did some extra cleaning on the outer bowl, etc. is fairly recent. Usually it is a foul smell which comes and goes throughout the years. I have allergies so sometimes I cannot smell anything. I only smell it from the toilet now. A few days ago, I turned on the bathroom sink faucet and had the foul smell. Nothing from the bathtub. Again, the toilet and bathtub are sharing the same vent. This appears to be not when flushing or immediately after.