Take a look in your dishwasher. You should see a little compartment for the detergent—and next to that, another little compartment. The second one is for the rinse aid, liquid stuff made up of surfactants and salts and acids. It’s designed to help your dishwasher work better, to give you cleaner and drier dishes that are all sparkly and pretty. Common concerns about rinse aid include whether it coats your dishes in gunk and hurts the environment, or whether it’s totally unnecessary. But a closer look reveals that it isn't dangerous and is actually very helpful—your dishes will never get as clean without it.

As much as we might like to believe the claim, rinse aid isn’t just a money grab for detergent companies.

You need rinse aid because dishwasher detergents don’t work the same as they used to. If you’ve read our guide to the best dishwashers, you know that in 2010 the Environmental Protection Agency and other regulators made detergent companies stop using phosphates, a great cleaning agent, because they can lead to algal bloom.

Says Liam McCabe in our dishwasher guide:

Every new dishwasher has a rinse-aid dispenser because rinse aid is essentially mandatory if you want your dishwasher to work well these days, according to every industry person we talked to. Rinse aid offsets the limitations resulting from gentler detergents and stricter efficiency standards—it’s just part of the deal now.

What’s in this stuff? And what does it do?

We’re going to use Finish Jet-Dry as our standard rinse aid, since that seems to be the dominant brand. (Disclaimer: Don’t take this as a recommendation! We haven’t tested any rinse aids, and we haven’t put this one through our normal Wirecutter wringer.)

Finish Jet-Dry rinse aid has a bunch of stuff in it, but it isn't complicated, really. Here's a rundown of the contents:

Water is necessary to dissolve all the other stuff.

Alcohol ethoxylate is a nonionic (uncharged) surfactant that helps the water slide off your dishes better and thus helps them dry faster. This ingredient is probably the most important bit in rinse aids; more on how it works in a minute.

Sodium polycarboxylate is an anti-redeposition polymer that wraps itself around the crud that the dishwasher just washed off so that the bits don’t get stuck again on your dishes.

Citric acid, which RB (the company that makes Jet-Dry) calls a complexing/sequestering agent, is really good at grabbing calcium ions out of hard water. Calcium can bind with surfactants and keep them from cleaning and rinsing dishes, so citric acid acts as kind of a sacrificial lamb to keep calcium from interfering.

Sodium cumene sulfonate is another surfactant but with an electric charge, so it’s a bit better at breaking water’s surface tension on your dishes than alcohol ethoxylate, but it’s also more foamy (PDF). Foam is bad in a rinse aid, so that’s why such products use both kinds of surfactant.

Tetrasodium EDTA is a chelating agent. EDTA is short for ethylenediaminetetraacetic acid. It’s this funky-looking molecule that wraps its four arms around dissolved minerals in the water (such as calcium). The word chelate comes from the Greek word for "claw," so you can imagine this molecule sinking its claws into minerals and whisking them away, similar to what citric acid does.

Methylisothiazolinone and methylchloroisothiazolinone (aka MI and MCI) are both preservatives, meaning they keep bacteria from growing in your bottle of rinse aid. Both are capable of causing skin allergies and are sensitizers, meaning that if you’re exposed to them over and over again, you can develop an allergy. But since rinse aid doesn’t sit on your skin and washes away completely from your dishes, I wouldn’t worry about it here.

CI Acid Blue 9 is dye. It makes the rinse aid blue. Why does it need to be blue? I have no idea, although colored solutions are easier to see in that little rinse-aid compartment.

As I mentioned above, the surfactants (short for “surface active agent”) are probably the most important part of rinse aid. Water is an oddly codependent molecule: It likes to hang on to its neighbors as much as possible. Water molecules on the surface freak out a bit because they don’t have any water molecules to hold onto above them, so they hang on doubly hard to their friends next door. This behavior is known as “high surface tension,” and it’s why water beads up on glasses and plates—the water would rather stick to itself than spread out on the plate.

Surfactants can break this high tension because they give those surface water molecules something they’d like to hang on to instead of their neighbors. So the water that used to bead up on your glass then spreads out in a thin layer. Water in a thin layer evaporates much more easily than beaded-up water, so your dishes come out dry at the end of the cycle.

Another major thing rinse aids do is prevent water spots on glasses. Remember the sequestering and chelating agents in there, citric acid and tetrasodium EDTA? These components grab the stuff that makes water spots—dissolved minerals such as calcium—and whisks them away. If they’re rinsed away, they don't stay behind in the water, so no spots. Magic.

The last really important bit is that anti-redeposition agent, sodium polycarboxylate. It keeps food bits in the wash water from ending up back on your dishes.

Why do you need more surfactants? Isn’t detergent made of surfactants?

Yes, dishwashing detergent contains surfactants. But it also has hella complexing agents and enzymes. Different surfactants are better at different things: Some are better at cleaning, and some are better at breaking up water’s self-bonding party. The latter are the kind that tend to be in rinse aids. If you’re thinking you’ll be smart and just use extra detergent, womp womp. If you put in extra detergent, not all of it will rinse away cleanly, leaving you with a film of detergent on your dishes. Too much detergent can even etch glasses. Don’t do it.

What if you don’t want this crap on your dishes?

According to the customer service rep I talked to at Finish, if you use rinse aid properly—that is, put your dishwasher on the hottest, longest cycle—no residue will remain on your dishes. That’s the ideal situation. But what about less-than-ideal situations? According to the 16-ounce bottle of Finish Jet-Dry rinse aid, it has 150 washes of stuff in there, or 0.1 ounce per wash.1

Assuming that no rinse aid gets rinsed off during the wash cycle, the concentration of rinse aid in the dishwasher water is about 0.0005 ounce per rinse. Divide that among all of the dishes in your dishwasher, and you get ... a really tiny amount. And at least some of it, maybe all of it, will get rinsed off.

What if you don’t want this crap in the environment?

As we say in our dish soap guide, water-treatment plants do a good job of cleaning surfactants out of the water. Two of rinse aid’s ingredients, alcohol ethoxylate and sodium cumene sulfonate, are considered low risks (PDF) to aquatic life. Another ingredient, tetrasodium EDTA, has a toxicity that’s a bit complicated because it almost always has some kind of ion (such as calcium) attached to it in water, which changes its chemistry. However, it isn't a big risk with normal home use (PDF). Everything else checked out as having low toxicity to aquatic life. So unless your dishwasher drains directly to a stream (who are you?), rinse aids seem to pose little environmental risk.

What about vinegar in a cup? Is that cheaper, and does it work the same as rinse aid?

Yes and no. People all over Internet-land suggest using vinegar instead of commercial rinse aid, but this approach has two problems. First, you should not put vinegar in the rinse-aid dispenser in your dishwasher. Vinegar is a strong enough acid to melt the rubber gaskets in the rinse-aid dispenser. Bad. Some people suggest running a rinse cycle with the vinegar in a cup on the top rack, and this tactic can work. It is a bit of a pain, since you need to stop your dishwasher and put the cup in just before the rinse cycle. It also doesn’t work as well as rinse aid. Vinegar can be a chelator, but it’s not as good as EDTA. It can also mess with the surface tension of water, but not as well as alcohol ethoxylate and sodium cumene sulfonate. So it’s cheap (cheap-ish, actually—working in such small amounts, it’s hard to say), but in this case you get what you pay for.

So do you really need rinse aid?

Well, no, not really. They’re your dishes, do whatever the hell you want. But if your dishes are coming out of the dishwasher wet, or with food bits still stuck to them, give rinse aid a whirl. Or try the vinegar-in-a-cup thing. Life is an experiment—play with it and find what works best for you.

When a source of light moves toward you, its waves are compressed and pushed to a higher energy. We can’t always see this blue shift, but it’s there.

In the space of Internet science, there’s a lot of bad information floating around. In this biweekly column, Leigh Krietsch Boerner, chemistry PhD and science editor of Wirecutter, will tell you what you need to know on the science of home products, and what’s all around you.

Footnotes:



1. Bosch, the company that makes our dishwasher pick, told Liam McCabe that 3.5 drops of rinse aid is used per dishwasher cycle. One drop is technically 0.05 milliliters, so that’s about 0.175 mL of rinse aid per each cycle. Since 0.1 ounce, the amount I used in the above calculations, is about 3 mL, somebody’s off here. I used the higher number because a too-high estimate is better than a too-low estimate in this case. We’re not exactly sure how much water your dishwasher uses in the rinse cycle, but we did find an older GE manual that says just running a rinse cycle uses 1 gallon of water. Dishwashers are different, so we’re guesstimating that the rinse cycle on most dishwashers uses about 1.5 gallons of water. One gallon is about 128 ounces, so 1.5 gallons is 192 ounces. Jump back.