How Do Smell Proof Bags Work? (And Do They Really?)

Quick answer: A smell proof bag doesn't seal smell in. It has a lining of activated carbon, and the carbon catches odor molecules as air moves through the fabric. The shell and the zipper slow the air down, and the carbon does the rest. That's why the bag keeps working after you've opened it a hundred times.

You've probably tried the sandwich bag already, because most of us have at some point. You zip it shut, squeeze the air out, drop it in a backpack, and an hour later the whole backpack smells like whatever was in the bag. And the annoying part is that the bag didn't leak. It's still zipped. So how did the smell get out?

That's pretty much the whole story of smell proof bags, so it's where we'll start. Once you can see why the sandwich bag let you down, the way a carbon-lined bag works stops being mysterious, and you'll also see where it runs out of road.

What Is a Smell Proof Bag?

It's any bag that's built to keep odor from getting out, which sounds simple until you notice that two quite different things get sold under the name.

The one most people mean is the carbon-lined bag. From the outside it looks like an ordinary wallet or pouch or backpack, and the difference is a layer of activated carbon sitting inside the fabric. This is the kind you carry things around in.

Then there's the mylar bag, which is a flat resealable pouch made from a metalized film, and it works in a completely different way that we'll come back to further down. You'll see both of them called odor proof bags or scent proof bags as well, and as far as we can tell nobody means anything different by those.

How Do Smell Proof Bags Work?

Here's the bit that surprises people. A fabric bag can't be airtight, and it was never going to be, because it has a zipper and seams and the cloth itself lets air through. So a smell proof bag doesn't even try to stop the air. What it does is clean the air on the way out.

The carbon layer does the work

Activated carbon is charcoal that's been treated until it's riddled with microscopic pores. And we do mean riddled, because a single gram of the stuff has more internal surface than a tennis court, all of it folded up inside a pinch of black powder.

Now think about what a smell actually is. It's molecules drifting around in the air, and when that air works its way through the carbon layer, those molecules stick to the walls of the pores and don't come off again. The air carries on out of the bag and the smell stays behind. Chemists call this adsorption, with a d, and it's worth half a second of your attention because it isn't the same as absorption. Nothing is soaking in the way water soaks into a sponge. The molecules are clinging to a surface, and there happens to be a ridiculous amount of surface in there.

Ours is activated carbon made from bamboo, and it's the same lining whether you're looking at the smallest wallet we make or the duffle.

The shell and the zipper slow the air down

There's a catch, though, which is that carbon can only grab what actually passes through it. If air can pour straight out of a gap, it just skips the lining. And where's the biggest gap on pretty much any bag? It's the zipper. An ordinary one is a row of teeth with daylight between them, and that's where the smell gets out of most bags that claim to hold it.

So a decent smell proof bag needs a tight outer fabric and a zipper that's been sealed or waterproofed. On ours that means a 1000D polyester shell, with waterproof zippers on the smaller bags and seam-sealed ones on the backpack and the duffle. We should be honest that none of this makes the bag airtight. All it does is make sure that whatever air does leave has to go through the carbon first.

Why it doesn't need to be airtight

This turns out to be the real advantage, and it's an easy one to miss. Think about a jar for a second. It works beautifully while it's shut, and then you open it and everything that's been building up inside comes out in one go. A carbon-lined bag isn't relying on a seal in the first place, so you can open and close it all day and nothing about it stops working. Which is how a bag gets used anyway, isn't it?

Why Doesn't an Ordinary Plastic Bag Hold a Smell?

So, back to that sandwich bag. It was shut the whole time and the smell got out regardless, and the reason is a bit odd when you first hear it: the smell went straight through the plastic.

A sandwich bag is a very thin film of polyethylene. It'll hold water, and it holds air well enough for a packed lunch, but small odor molecules can work their way into the film and come out the other side. It happens slowly, which is why the bag seems fine for the first few minutes and then gives you away after an hour in a warm backpack. A thicker freezer bag will buy you some more time, and so will putting one bag inside another, but you're only ever slowing it down.

Glass and metal are a different matter, since nothing gets through the wall of a jar at all and the only way out is past the lid. That's why a jar with a good gasket will hold a smell that a plastic bag won't, even though the two of them look equally closed. A good jar keeps flower fresh for longer too, which is its own topic: does flower go bad.

Three Ways to Hold a Smell

If you put all of that together, there are really three ways of going about it, plus the one that doesn't work, and they're good at different jobs.

Mylar is the one we skipped past earlier. It's a polyester film with a thin layer of metal in it, and odor molecules can't get through that the way they sneak through a sandwich bag. So where a carbon bag catches the smell, a mylar bag simply blocks it. Our Color-Coded Mylar Bags are 120 micron with a heavy-duty resealable zipper, and we'd use them for portioning herbs, spices and loose leaf tea, since they're not really something you'd carry gear in.

In practice most people end up using two of these at once, with a jar or a tube sitting inside a carbon-lined bag. The container looks after whatever's in it, and the bag takes care of everything else you've got with you. If it's the container half you're trying to sort out, our guide to smell-proof storage goes into jars, tubes and what airtight really means.

Do Smell Proof Bags Actually Work?

For everyday odor, yes, they do. A bag with a real carbon lining will keep a room, a car or the person sitting next to you on the bus from noticing what's in it, and it'll go on doing that however many times you open it.

But "smell proof" gets thrown around pretty loosely, so it's only fair to tell you where it stops.

The first thing is that the bag has to be closed. It sounds too obvious to say, but while the zipper's open the air inside goes straight out and the carbon never gets a look at it. Overfilling does something similar, because once you've stuffed a bag until the zipper strains, you've made a gap that air can use to skip the lining.

Then there's the lining itself, which has to be the real thing. A thin or decorative layer won't do much whatever the label says, and since the carbon is what you're paying for, that matters. It also fills up over time. Every pore that's holding on to a molecule is a pore that can't take another one, and we'll get to what you can do about that in a minute.

It also can't do anything about smell that's on the outside. If your hands carried some onto the shell or the zipper pull while you were packing, that's what people are going to notice, and it's easy to blame the bag for it.

And it isn't dog proof. No bag you can buy can honestly claim to beat a trained detection dog, and we'd be wary of anybody who promises you that.

What a good bag can do is contain normal odor in normal situations. We think that's the realistic standard, and it's the one ours are built to.

How to Test a Smell Proof Bag at Home

You don't have to take anybody's word for any of this, ours included. Put something in the bag that smells strong and is harmless, like fresh ground coffee or half a cut onion. Zip it all the way, and have a look that the pull is right at the end of its track, because half an inch of open zipper is enough to ruin the test.

Then leave the bag in a small closed space for a few hours. A cupboard is ideal, though a car with the windows up will do just as well. When you come back, smell the space before you go anywhere near the bag. If the cupboard smells of coffee, the bag has failed, and if it just smells like a cupboard, you're fine. It's quite satisfying to run the same test with a sandwich bag alongside, by the way.

Is Airtight the Same as Smell Proof?

No, and we think mixing the two up is how a lot of people end up disappointed with what they bought.

Airtight means that no air gets in or out, which is true of a jar with a gasket and will never be true of a bag. Smell proof only means the odor doesn't get out, and you can get there either by being airtight or by catching the odor on carbon. Odor resistant is the one to watch out for. It usually means a coating or a tight weave that slows the smell down for a while, and there's no carbon in there doing any catching.

So a jar is smell proof for as long as it stays shut, and a carbon-lined bag manages to be smell proof without being airtight at all. A bag that's only described as odor resistant probably isn't either of those things.

How Long Does the Carbon Lining Last?

Longer than you'd expect, though it won't last forever. The carbon keeps holding odor molecules until its surface is full, and how quickly it gets there comes down to how you treat the bag. If it's packed full of something strong and left zipped for months, it's going to slow down sooner than a bag that only gets light use.

You can help it along a bit. Empty the bag when you're not using it and leave it open somewhere dry, so it gets a chance to air out. You'll also come across advice online about warming the lining with a hair dryer to refresh it. We haven't tested that on our own bags, so we're not going to tell you it works, and airing it out is the part we're actually sure about.

How do you know when it's finally spent? Run the cupboard test again. If a bag that used to pass doesn't any more, the lining is full, so give it a few days of airing and try once more. If that changes nothing, it's time for a new one.

Can you wash a smell proof bag?

Not in a machine, and we wouldn't soak it either. Water and detergent are bad news for the carbon layer, which is the one part of the bag you can't replace. Wipe the outside with a damp cloth, spot-clean the inside if you need to, and let it dry with the zipper open.

Can You Make a Bag Smell Proof Yourself?

Not really, although you can make an ordinary bag a good deal better than it is. You can't add a carbon lining at home, and no amount of double-bagging is going to turn polyethylene into a barrier, so that route only ever buys you time.

What does work is putting something airtight inside the bag. An airtight jar or a metal storage tube with a proper seal holds the smell right at the source, and at that point it hardly matters what the bag around it is made of. The weak moment is when you open the jar, and that happens to be the gap a carbon-lined bag is there to fill.

What to Check Before You Trust One

Say you're looking at a bag and wondering whether it'll actually do the job. You can tell most of what you need from three things.

Start with the lining. It should say activated carbon in so many words, and a maker who's proud of it will usually tell you what it's made from, which in our case is bamboo. After that, look at the zipper, and what you want to see is waterproof or seam-sealed, because an ordinary toothed zipper is exactly where the smell gets out. The shell is the last thing. A dense fabric like 1000D polyester keeps air from simply pouring through the sides.

A lock is worth having as well, but it's got nothing to do with odor. It's there for a bag that lives in a shared house or a locker or a car, and for keeping kids and pets out of things. We've laid out the sizes, the locks and which bag suits which kind of trip on our smell proof bags page, so we won't go through them again here.

Smell Proof Bag FAQ

How do smell proof bags work?

They catch odor on activated carbon. The lining is full of microscopic pores, and odor molecules stick to them as air moves through the fabric. A tight shell and a sealed zipper slow the air down so it passes through the carbon. The bag isn't airtight, and it doesn't need to be.

Do smell proof bags really work?

For everyday odor, yes, as long as the lining is real activated carbon and the bag is zipped shut. They stop a room or a car from noticing what's inside, and they keep working through repeated opening. No consumer bag can honestly promise to beat a trained detection dog.

What is a smell proof bag made of?

A carbon-lined bag has a dense outer fabric, a layer of activated carbon and an inner lining, closed with a waterproof or sealed zipper. ONGROK bags use a 1000D polyester shell over activated bamboo carbon. Mylar bags are different. They're a metalized polyester film with a resealable zipper.

Why aren't plastic bags smell proof?

Because odor passes through the plastic. A sandwich bag is a thin polyethylene film, and small odor molecules slowly work their way through it even when the bag is zipped. A thicker bag or a second bag slows that down and doesn't stop it. Glass, metal and metalized film don't let odor through.

Is airtight the same as smell proof?

No. Airtight means no air moves in or out, which is what a jar with a gasket does. Smell proof means odor doesn't escape. A carbon-lined bag is smell proof without being airtight, because the carbon catches the odor as air passes through.

How long does a smell proof bag last?

It depends on use. The carbon holds odor until its surface fills up, so a bag that's packed full and stored closed for long stretches slows down sooner. Emptying it and airing it out somewhere dry between uses helps it last.

Can you wash a smell proof bag?

Don't machine wash or soak a carbon-lined bag, because water and detergent can damage the carbon layer. Wipe the outside with a damp cloth, spot-clean the inside and let it dry open.

How do you test a smell proof bag?

Put something with a strong smell inside, like ground coffee, zip the bag fully and leave it in a closed cupboard or car for a few hours. Then smell the space, not the bag. If the space smells of what's inside, the bag isn't holding it.

Can you make a bag smell proof?

You can't add a carbon lining at home, and double-bagging in plastic only slows odor down. The fix that works is an airtight jar or metal tube inside the bag, which holds the smell at the source until you open it.

Do smell proof bags work for weed?

Several customers report that our carbon-lined bags work extremely well for that, but it isn't the intended use. ONGROK bags are made for dry herbs, gear and anything else you'd like to keep to yourself. The lining works the same way whatever is inside. Check your local laws before you carry anything.

ONGROK products are not meant to be used for any illegal purpose.