Add a device called by one trade name of “Pro-Vent”. This a one way air check valve that will allow air in and check gases from coming out i nthe living space. It is allowed by a lot of code regs in the USA.
A Provent also known as an Air admittance valve AAV is NOT legal in most civilized codes.
The reason this (IMHO non professional garbage) is not allowed is certain code officials frown upon sewer gases from entering the home. Sewer gas cannot only make someone sick BUT it can also cause explosions and some fumes are very toxic and will cause death.
These AAV’s depend on as cheap rubber flapper to close after it opens to allow the ambient air to enter the drainage system.
Now I am certain anything Mechanical WILL fail after a period of time OR in this case a bug climbing out of the vent line and caught in this cheap garbage flapper will render it useless.
Another secrete no one talks about is rubber can dry out a become brittle and again this will cause a failure in the OPEN position allowing all kinds of great smells into your living area.
A lot of guys try to sell these devices because they are CHEAP
If you contact a local plumbing contractor they can perform either a peppermint test OR a smoke test to find out if indeed your vent is working.
A peppermint test is the easiest test to try. My 1st posting said keep AWAY from the AAV (provents) but hey its your health. Good luck Sylvan