If you’ve snooped around your grandma’s attic or  wound through the shelves at a vintage bookstore,   then you might be familiar with mothballs.

Or rather, their smell. They have a sharp, overwhelmingly sweet,  tar-like scent reminiscent of old tobacco smoke.

It’s a smell so distinct that I bet  you’re wrinkling your nose right now. And it’s also why we’re not opening this bag. But that smell, and really mothballs in  general, are becoming far less common.

We just don't really see—or  smell—them much anymore. For more reasons than you might think. [♪ INTRO] For those of you who haven’t encountered  these pungent little balls before,   they’re intended use is to deter moths. Except “moth ball” is  actually a bit of a misnomer.

It’s not the moths that are  the problem, it’s the larvae. There are several species of clothes moths that  can become household pests, but all species are   super into the protein-maxxing trend, and  their protein of choice is fibrous keratin. And what better place to chow down than  our closets full of keratin-rich wool,   silk, fur, and leather.

They’re pretty shy about their eating  habits, and by that I mean they often   hide in the protected nooks of clothes,  or the backside of furniture and rugs. Plus, the larvae are translucent when  they’re first born, making them hard to spot. So usually your first sign of an infestation  would be lots of new holes in your clothes,   which happened to me a couple of months ago!

Eventually they’ll build a cocoon out of silk  from their bodies as well as bits of fabric. Then they’ll lay in wait to  transform into an adult moth,   who lays eggs and the cycle starts again. But our closets aren’t the only  places that clothes moths hang out.

Keratin is also in vintage bookbinding  glue and leather book covers. Museums are also prime targets, with their massive  collections of keratin-full preserved specimens. And clothes moths are one of  our most ancient nuisances.

Even Alexander the Great’s housekeeper had   to be diligent about brushing  clothes to dispel the moths. So you could say for a while now people have  been pretty keen on getting rid of these critters. In the past that mostly meant natural deterrents,  like chips, chests, or hangers made from cedar.

Which, is lovely smelling, but there’s mixed  evidence about whether it actually works. Cedar fumes can kill very young  larvae, but research has shown   they unfortunately aren’t effective  against older larvae or adult moths. Plus, cedar lumber eventually loses its  essential oils that smell so nice to humans.

And within a few years the vapor concentrations  aren’t high enough to deter even the young larvae. Then there’s the added downside that, over time,   cedar can become acidic and leave yellow stains  on the fancy clothes you’re trying to protect. And there seems to be a bit of a smelly  theme when it comes to deterring moths.

Other popular smelly options are oils like  lavender, eucalyptus, rosemary or camphor,   an oil distilled from the camphor tree, a  large evergreen tree native to East Asia. You probably know what those first  three smell like, but in case you’re   not familiar with camphor, it’s a sharp,  nostril-opening, almost turpentine smell. It’s an ingredient in Vicks Vaporub and Tigerbalm.

Although to be fair those both have a bunch   of additional smelly compounds as  well, like menthol and eucalyptus. All of these are quite different scents  but one thing they have in common,   chemically speaking, is that they contain  terpenoid compounds — organic molecules   made from links of a 5-carbon  structure called isoprene. Terpenoids are pretty common in a lot of plants,   and they seem to disrupt the nervous  and digestive systems of some moth   larvae… but unfortunately don’t protect  against the moths that eat your clothes.

A recent study at least found that, as a gas,   camphor does deter red ants by disrupting  their climbing and feeding behavior. But again. No moths.

So after many years of a battle against the bugs  with smelly substances of… questionable substance,   you can imagine how satisfying it  must have felt to finally discover   something that was proveably effective  against those wool-munching nuisances. Enter mothballs. Which we’ll talk about in detail  right after this quick break.

Since you watch SciShow, you  probably like engaging with   new ideas and learning outside of a classroom. And that’s what makes this video’s  sponsor, Brilliant, so great too. Brilliant is an online personal tutor that offers  guided courses in math, science, and coding.

Brilliant’s tutor, Koji, can give your kids the   one-on-one attention that  you wish you had in school. Koji can draw on your screen,  break down problems step by step,   and keep asking questions until  a student understands a concept. To get started with Brilliant’s tutor for  free, click the link below or scan the QR code.

You can upgrade to Premium to unlock all courses. And right now, SciShow viewers can save 20% off  an annual subscription at brilliant.org/scishow In 1948 chemists standardized the  production of a white crystalline,   volatile solid chemical called naphthalene  aka the main ingredient in mothballs. This was a long time coming though,  since naphthalene was first discovered   all the way back in 1819 by a Scottish  chemist when he was distilling coal tar.

Which is actually not too different  from how naphthalene is made today. Some 90% of naphthalene is made by burning coal  tar at super high temperatures and cracking   the chains of molecules apart into the fused  pair of benzene rings we know as naphthalene. It’s also made during natural combustion  processes like burning wood or tobacco.

Chemists later created a more effective compound   called paradichlorobenzene. And  this one was kind of by accident! While making explosives for the First World War,   chemists noticed they’d made  a strong-smelling byproduct.

They passed it along to entomologists,  who then made it into an insecticide. Paradichlorobenzene also has a benzene component,   as the name suggests, except this compound  is synthesized by adding chlorine gas. Again, as the name kind of suggests.

And as its history suggests,   this clever little compound can be  used in a couple of different ways. Anyone who’s spent time around  urinals might recognize the smell   of paradichlorobenzene because it’s  the main compound in urinal cakes. If you’re not familiar with the smell,   you can pause the video and take a trip to your  nearest public bathroom and learn for yourself.

As well as bathrooms, you might  find it in your closet or under   the kitchen sink as an odor eliminator block. Its deodorizing ability means it  also shows up in cleaning products,   leaving you with that super clean smell. But it’s also a potent fungicide  that helps control plant diseases.

And it’s an insecticide against not just  our little moth foes but also silverfish. But what is it about naphthalene  and paradichlorobenzene,   over cedar or camphor, that  make them so effective? If you’ve been following along, you  might have noticed a theme here: Smell.

Part of the reason these substances are so   infamously smelly is because they  have pretty low odor thresholds,   meaning we notice the smell even when only a  few of the molecules make it up our nostrils. So, that’s part of the answer. Another is that they sublimate easily,  meaning they turn straight from solid to gas.

They’re able to do that because the  compounds have a high vapor pressure. Basically the molecules of the  bulk substance push against the   air more strongly than atmospheric pressure,   lowering the energy barrier for molecules to  peel off the solid and change states to a gas. It’s that gas that works as a pesticide.

But maybe most importantly, those  gaseous compounds don’t just deter   but can actually kill larvae and adult moths. Now, there’s not much information on how they  work, but one study on mosquitoes found that   naphthalene might interrupt the functioning  of the larva’s respiratory organ, or siphon. Trouble is, they may be too good at their job.

Naphthalene and paradichlorobenzene have turned   out to be almost as bad for  humans as they are for bugs. We started to understand the health  risks through good ol’ toxicity   testing on animals but also several case  studies where people, especially children,   had accidentally eaten a mothball or two. Because of those cases some countries,  like New Zealand, don’t allow them at all,   since there’s too much of a risk of  children being poisoned by eating them.

And it’s not just children. There have been cases of  adults ingesting them too. Whichever way you come into contact  with naphthalene from mothballs,   its effect is pretty similar.

In the body, naphthalene is broken down into  alpha-naphthol and then other compounds,   which can interfere with enzymes or damage cells. Red blood cells explode, known as  hemolysis, which can lead to anemia. It also damages the lining of  your lungs and mucus membranes.

At first that looks like inflammation,  but after a while they can kind of end   up permanently swollen –  at least in mouse studies. If that’s not enough,  naphthalene is also carcinogenic. Paradichlorobenzene is a little less toxic,  causing eye or throat irritation when it’s first   inhaled, but also ultimately ends up damaging  your blood cells, liver, kidneys, and sometimes   central nervous system, leading to weakness,  slow reflexes, trembling or slurred speech.

And even camphor, although it seems more natural,   can irritate the esophagus,  stomach, and liver if it's eaten. It can also lead to vertigo, confusion,  delirium, seizures and coma in young children. Remember, natural does not always mean safer!

By the late 1990’s it had become pretty  clear, through experiments and formal   toxicology reviews by agencies like the  Environmental Protection Agency in the US,   that naphthalene was bad news for our  health and not worth a few sweaters. Which meant mothballs started coming  with some stern warning labels. They’re still available as pesticides,   but there’s also plenty of other things  we can do to help keep moths at bay.

For example, cleaning your clothes in  hot water and then storing them in an   airtight container is usually enough to  make that woolen sweater unappetizing. Larvae will actually seek out soiled patches  on clean garments, so the cleaner the better. It also helps to bring that sweater out  every month and give it a little shake.

Clothes moths love that chill  life and hate being disturbed. The same goes for carpets. Regularly vacuuming and cleaning them gets rid   of the lint and animal hair  that larvae love to feed on.

And walking on those carpets  also helps disrupt the moths. If there is an infestation, there are  other, less toxic ways to deal with it,   like pheromone traps or water-based insecticides. Or, just put the clothes in the freezer  for two weeks inside a sealed bag!

Easy! But those fuzzy sweaters and cozy carpets  may not even need that much saving anymore. Not only are mothballs a thing of the past,  but so, to some extent, are moth infestations.

Climate control keeps our homes relatively cool  and dry, where bugs like things warm and wet. Plus it means we keep our  windows and doors closed,   so less chance of moths  getting in in the first place. The few places clothes moths might still  be a problem is in more humid regions   or in older houses with lots of lovely  crevices for moths to creep in through.

But just like our homes have changed,  our clothes have changed too. It’s far more likely to see rayon or polyester   printed on the tag than it  is to see 100% pure wool. Even a lot of animal fibers these days are blended   with other stuff to get the stretch or  softness benefits of synthetic fibers.

Which is good, in this case at  least, since moth larvae just   can’t digest synthetic fibers or even some  plant-based proteins like cotton or linen. So that distinct mothball  smell might be on its way out. Although maybe you’ll still catch a  whiff if someone wearing a vintage   coat stands behind you while  in line at the flea market.

Clothes moths no longer plague us the way they  did for centuries because we’ve changed the moth’s   environment through heating, the way we wear  clothes, and what those clothes are made of. We’ve narrowed their niche, so now holey  sweaters are nearly a thing of the past. [♪ OUTRO]