Sunlight Experiments

I have always been afraid to take my children to indoor playgrounds. The first time I took my son to an indoor playground he caught the terrible stomach bug, rotavirus. However, I have always enjoyed taking my children to outdoor playgrounds. I have always felt that they were cleaner with the sun and the rain washing them. Yes, outdoor playgrounds can have dirt which is full of bacteria and germs. However, the germs in dirt are generally good immune system building germs. Of course, I tried to keep my kids from eating dirt, but all of my toddlers have eaten dirt numerous times and nothing horrible happened. Yes, there are some terrible illnesses that one can catch from dirt, but they are far less common than the nasty illnesses that you can get from other people, indoor playgrounds, public bathrooms, and the pediatrician's office. After many requests, I've decided to test how well sunlight kills germs.

Does hanging laundry out to dry in the sun kill germs?

After I did a bunch of laundry experiments (which you can see here), I had many requests to determine if hanging laundry out in the sun would kill germs. To do this, I took all my dirty kitchen towels and washcloths which had been sitting in a laundry basket for a few days and numbered 7 of the washcloths with permanent marker (1 through 7). Then I washed the washcloths with the rest of the kitchen stuff on cold in my Samsung high efficiency top loader. I do not recommend this machine. Look how little water it used for these cloths. They were barely agitating. What you see here is really germ soup. I used Tide free and clear detergent. I did not use bleach.

After the 53 minute wash cycle, I dabbed each "clean" washcloths onto its own agar plate (which bacteria love to grow on). You can purchase some of these agar plates for your own experiments here on amazon

Then I put cloths numbered 1 and 2 in my Samsung HE dryer on the sanitize setting for 1.5 hours with the rest of the kitchen laundry. I do like this dryer pretty well. 

I hung cloths numbered 3 and 4 up to dry in my computer room for 4 hours. Please ignore how messy this room is. 

I hung cloths numbered 5, 6, and 7 outside on a hot sunny day (88 degrees F) for 4 hours (from 2pm to 6pm). There was hardly a cloud in the sky. Halfway through the drying, I moved the clothes line so the opposite side of the cloths was directly facing the sun. So, both sides of the cloths had time in the sun. I did notice that some bugs landed on the cloths which could bring more bacteria to them.

After the specified time, I blotted each cloth onto another agar plate. I wet the cloths with a little sterile water before I blotted them onto the agar plates because the germs transfer better when they are damp. Then I incubated the cloths for 24 hours in my warm incubator (about 90 degrees F). 


As you can see, the washcloths were disgustingly FILTHY and full of bacteria after going through the washing machine. Yuck. The sanitize setting on the dryer killed most of the bacteria. In case you don't know what you are looking at, each white or yellowish dot is a colony (or pile) of millions of bacteria. 

The cloths that hung up to dry in the computer room were still disgustingly filthy after hanging up to dry inside for 4 hours. 

The cloths that were hung outside in the sun were much cleaner than the ones that hung up inside. Maybe not quite as clean as the cloths that went through the dryer, though. 

Just as I had hoped, hanging cloths out to dry on a sunny day killed the majority of the bacteria! It certainly could be possible that bugs that landed on the cloths and dirt blowing in the breeze could have added a little bacteria to the cloths as well. So, if you like to hang your laundry out to dry in the sun, go for it! It really reduces the germs! 

Does the sunlight kill germs on hard surfaces? 

I also wanted to know if bacteria on a hard surface would be killed by sunlight. To test this, I used 2 white ceramic plates. I sterilized the plates by spraying them with 3% hydrogen peroxide and wiping them off. Then, I labeled one "sun" and the other "dark". I mixed up germ water using a little scoop of dirt from my back yard and mixing it with water. I poured it through paper towel to remove chunks. 

I put 1mL of this germ water onto each plate and rubbed it around with my gloved hand. Then I let the plates dry completely inside the house which took about 2 hours. 

Once the plates were dry, I stacked them up. The "sun" plate was on top and the "dark" plate was on the bottom.

Then I took the plates outside on a completely sunny blue sky day. It was about 80 degrees F. I set the plates on my patio table on top of white paper towel, because I didn't want the plates to get too hot. I covered the edges with white towels because I didn't want sunlight sneaking into the cracks between the plates. The reason that I stacked the plates as opposed to keeping the "dark" plate inside the house was because I wanted the plates to be at the same temperature. 

After 4 hours of sitting out in the sun, I used a sterile swab (wet with sterile water) to rub each plate and then rubbed the swab onto an agar plate. 

The agar plates were incubated for 24 hours in my warm incubator (about 90 degrees F).


As you can see, there is a significant decrease in the amount of bacteria on the plate that was exposed to the sun. I repeated the experiment a second sunny day and got the same results. 

I hoped that almost all of the bacteria would be dead on the plate exposed to the sun. However, the brown dirt on the plate can act as a little "sunscreen" and protect some of the bacteria from the sun's rays. 

Sunlight Conclusions

From this evidence, I think it is fair to say that sunlight kills germs.  However, I want to be honest and tell you that I didn't do every necessary control to be sure that no other phenomenon occurring outside on a sunny day is responsible for the bacteria killing that I saw in these experiments. I didn't do a control to make sure that the breeze wasn't responsible for the germ killing. The control I need would be to hang the washcloths outside on a hot sunny day for 4 hours (and put the plates outside on a sunny day for 4 hours) but somehow have them completely shaded from the sun but not from the wind and any other outside forces. I've tried doing this with strategically placed umbrellas but somehow the sun always snuck in. I don't think this control is worth the trouble. UV light is known to kill germs so I am now a believer that the sunlight really does kill germs. Here is an article saying that sunlight was effective at killing some norovorus. However, if someone in your house has a stomach bug or other terrible illness, I wouldn't recommend using your normal cleaning cloths to clean up the mess because you can't completely trust the dryer or the sun to kill all the germs. I recommend cleaning up with these Clorox Hydrogen Peroxide Wipes

 that have been tested and kill norovirus. I always keep a can of these wipes on hand and hope that I never have to use them.