You mean hydrogen chloride gas and other PVC-cutting by-products? Chlorine itself is pretty harmless unless you are exposed to an acute dose - and you will smell it long before it reaches that level 
I am more concerned about some of the by-products of wood and leather cutting, since you often canāt smell them and some are genuinely hazardous.
The toxicity of using PVC in a laser cutter is only one aspect. The HCl gas thatās generated can combine with the H2O vapor by-product of combustion, and form a corrosive acid mist/vapor inside your lovely laser cutter.
You really donāt want to do that. 
What I meant was anything with āCLā as a primary element. You can call it Chlorine, Chloride, or infer it as part of Hydrogen Chloride. Any way you choose, I plan to avoid āCLā in all forms.
iāll point out for everyone else; avoid all halogenated materials unless you know beyond the shadow of a doubt that itās okay. no chloro-, fluoro-, bromo-, etc.
Indeed, I would certainly agree with that. Health effects aside, itās not good for your laser!
In the context of filters, Iām generally more concerned about things like chromium from leather which donāt always have an obvious odour and which do have dangerous side effects (i.e. cancer)
Iām still unsure what I will do with my setup - probably vent to an outdoor filter similar to what you have proposed (although Iām more inclined to simply buy one than build). Ideally if I have negative pressure inside the vent until itās outside that should prevent contamination of my living spaces, and filtering it outside will reduce contamination of the garden, neighbouring properties etc.
Well Iāve only done a bit of Googling on Chromium after reading your post, so Iāll preface this by saying Iām pulling this together on what little Iāve read, butā¦
Some articles I read indicated dilution reduction, which would basically be flushing the immediate area with lots of air. Not really filtration. OSHA compliance basically says Hexavalent Chromium should immediately be vented outside and not even captured.
Glowforge has mentioned that they have a few things āin the hopperā regarding keeping the fans running after jobs, letting people control settings, etc etc. Iād especially avoid Chromium items until Glowforge has an option to extend the fan runtime until long after the jobs are done.
personally iām hoping for something like an āadd 00:00:30 to fan timeā or the equivalent, just so you can really fully exhaust / pull away some of the odor
I was just thinking about this and wondering whether you can do zero-power really slow engraves ā add one of those to the end of each job.
Interesting concept. The UI will not go forward unless there is at least 1% power specified, but when we get the low power settings back that might be a really low number.
Ok, since I got such great technical information on my filter question here is a different idea. I do not have windows in the downstairs of my towne house that open, except for the small window over the sink that is already venting an air-conditioner, but I have a fireplace. Any ideas on how I use the fire place to vent or if I shouldnāt for some reason? And there is no place to put it upstairs.
theoretically this is fine if you run a chimney up through the fireplace - and of course, donāt use the fireplace. but youāll want to get a professional in to install it. it might be against code, but it might also be okay if you specifically arenāt using the fireplace (because youāve converted it into your laser vent stack).
Yeah, most of what @jrnelson said ā we donāt know what your intentions are regarding collaboratively using the fireplace and the Glowforge together. Youād want to have the exhaust above the damper for sure, though.
yeah, the big issue is that you need to vent it all the way up and out of the chimney, and properly cap it. for a run like that, youāll probably need to install a booster, but i canāt say for sure.
We wouldnāt use it as a fire place. And it will only be until the filter arrives.
then itās something you canāt really do. the only way to safely do it is to install a chimney inside of your fireplace that reaches to the roof, and probably boost it. otherwise it would just sink back down inside and fill your room.
No fires just art
If they used something like 2" thick pink foam insulation board, removed the damper and made the foam a tight fit and then taped all seams with foil duct tape it would likely work (as a non-permanent option while waiting on the filter).
if you say so. iām going to bow out here. i think itās a bad idea and more to the point, unsafe. but it is your laser and your chimney.
quick edit: i think itās a doubly bad idea if your townhome is connected to other houses. itās a potential fire risk.