Best bags to contain chemical odours? I have noticed that if you put household or other chemicals in bin liners or freezer bags the plastic doesn't keep the chemicals in. Rather they travel through the bag material and out into the air. However I have noticed that some bin liners have stronger material and are better than others.

Perhaps it will depend on which chemical is in the bag. However I guess I am asking about pollen, pesticides,washing detergent and general cleaning chemicals.

Is there a specific type/grade of plastic bin liners that I can get that will, generally speaking. keep odors in? Actually it doesn't have to be pvc any bag material which does the job will do however I guess PVC bags are most easily accessible so would like to know what options there are. See through bags are preferable.
As you state, the ubiquitous polyethylene (or polythene) bag allows many chemicals to slowly seep through. Pollen and dust particles themselves are far to large to get through, but water vapor, for example does.

For that reason, many foods are sold in plastic (e.g., mylar and polypropylene) sacks layered with aluminum (ah, aluminium for you polythene-sayers) to block evaporation, oxygen (which leads to fats and oils becoming rancid) and loss of flavor.

For storing small amounts of food, reuse these aluminized bags -- which also are effective shields to block RF tags in passports and licenses. They're available commercially, e.g. this list.

These bags are comparatively expensive, though, and last millenia in landfills, and impractical for general use waste bin bags.

Non-chemical work-around: store food wastes and other potentially odorous items in paper or plastic bags that would have been discarded anyway, and that bag inside the bin liner, to delay odor migration.