I’d like to take care of ventilation for my first grow using just an exhaust fan (4” for 4x4 tent) with passive air intake through ports in the tent. Total diameter of the passive intake ports available more than enough to work with the fan. I can coordinate exhaust fan operation for positive flow to meet the exchange requirements.
My question is, do folks use some sort of filter or screen to cover the passive air intake ports to safeguard against pulling anything undesirable into the tent?
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stockings 
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I don’t. My tents are in my house. If I can breath it I figure it’s good enough for the plants, lol.
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I put inline bucket diy swamp cooler to 8" bottom intake and i have less chance of flies when using it compared to my smaller tent out that has intake screen closed port so yes it helps.
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I bought myself a a bag of poly batting SHEET, not poly batting that is to stuff cushions.
I cut a piece to fit across the passive intake. I also picked a package of small round sticky back velco tabs. I then place one on each corner, of the passive and stick the poly bat to it but many ways will work.
You will see it collect, a dark area, in about 4-5 weeks of dust being caught.
Are they necessary, maybe not, but I know when we come into our grow areas from outside, we carry things in on us, them wee bitches are what I want to catch.
Wanna go nuts with it, smear a bit of Tangle Trap on the poly, but you will change them more often. Without TT, I simply rinse them under water and reapply.
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Best things probably just to staple or duck tape some ac filters over the mesh screen.
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I just use the mesh screen that comes with the tent and vacuum it out every couple of runs. Gets a bit dusty.
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Great ideas @the_bot, @webeblzr and @EugeneDebs420.
My plants are going to be in my house too @DougDawson but with furry friends afoot just want to make sure clean air is reaching the plants!
Another question I have about passive intakes is how to stop light penetration into the tent through them. How to address that? Placing a short run of ducting turned 90 degrees down towards the ground on the ports being used for passive intake and covering unused ports with cardboard or some other material?
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Fair enough, all my passive intakes have screens built in so I just don’t concern myself but I get it. You can always make a cardboard baffle to block the light or as you mentioned, a short run of ducting.
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Horti-Control Dust Shroom HEPA Filter, 6 in
In stock
$95.95
I had a piece of hard pipe connected to the flex pipe and put a car wax bonnet over the top of the hard pipe…Fuck $100 for a filter right… 
This is on the intake side BTW…
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Passive intakes definitely let a bunch of light in, you could make some kind of baffle to put over them or you could run autos(light pollution doesn’t affect them) or ideally you put it in it’s own room and block any light from the window/s and stay out of it during lights off(that’s how I do my flower tent) one of my cats is in the doghouse for going into my veg tent when I was filling my watering can.
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I use both passive and active but for my passive I put light baffles made out of blackboard from the dollar store for the metal screened rectangle ports on my tents and as for the 4-6" circular openings I use only vent ducting formed into a “u-turn” to allow airflow IN but not light. 
Example of a baffle:
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Holy shit lol. You know filtered duct screens are like $20 and are literally the same thing.
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Those are pretty cool. There is an oil impregnated into the filter which improves the filtering efficiency significantly. But yeah, I don’t understand the pricing either.
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Where were you several years ago, when I needed one…LOL
I did not know they made those, thank you very much.
I do not need one now, but dam if those aren’t pretty darn cool!
That is why we come hear to learn and have fun right?
Havin’ some fun, doin’ some learnin’ right now!
Thanks again!
Shag
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That is bad, cat in a doghouse, there has got to be a law…said the cat! 
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I swear my brain has like a built in auto compare for purpose built stuff. The first thing I think when I see something expensive and specific is what’s a general use equivalent.
There is definitely all kinds of dust and hairs and a bunch of other stuff the naked eye can’t see that blows inside my tent so
I use this carbon filter that cost $13 and cut out a 7 inch square and then use a clamp to hold around my intake fan.
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What drives the use of both passive and active @Pigeonman. Thanks for the diagram. And I intend to use the vent ducting in the circular ports as well.
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To the original question, yes you can filter the incoming air on your passive intakes, can be a simple modified house filter “basic you dont need a hepa”, cotton filter batting, duct filters, or said foam options above which is similar material to what i use for my cabs just a heck of alot cheaper.
The other consideration is if you have say those cuddly fur balls running around and or growing in a dusty area or near say your dryer, then filters probably are a wise choice otherwise you don’t need to run them.
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