The New York times has a fascinating article about EMF danger in hybrid cars. As a physicist who got his advanced degree in the town Einstein was born in, I would like to offer some comments and solutions to victims of this vile new thread to people who put their own physical and mental health (and maybe life) on the line to save the planet.
First of all, it is somewhat comforting to know that DC engines and batteries by their very nature are much weaker emitters of EMF per mass unit than for example radar stations or the 4p->alpha reactions found in stellar bodies. But it is important to keep in mind that a typical hybrid car battery has a reactive mass that is thousands of times larger than a cell phone battery--and we've all seen studies that indicate that cell phones can cause brain cancer, autism, or general malaise.
So, what is an environmentally conscious person to do? Risk life and limb every time s/he gets into a Prius or Chevy Tahoe Hybrid?
Thankfully, there are some fairly straightforward solutions. Most bio-relevant EMF fields can be effectively dampened by water, or a wetted fabric. For a good simulation, there is still nothing better than watching the animation about 10 minutes into the futuristic movie Total Recall. Notice how the agent dampens the electrical signal from the tracking device implanted in his head by wrapping a wet towel around his head. The same approach would of course work with a hybrid car battery, or a cell phone. But while putting a wet washcloth (or even a sanitary hand wipe) between the cell phone and your ear while making a call reduces the risk of EMF induced brain damage by 87% per 100 micron, wrapping a car battery in wet towels is both impractical and potentially dangerous.
All is not lost, because some elementary research online shows that the effect is caused not by the water itself, but by the dissolved salts. It almost sounds too good to be true, but if you are skeptical, you can verify it easily with some distilled water, a bit of salt, and a simple multimeter like this one at American Science.
Simply fill some distilled water into a glass and inset the two probes about 3 inches apart. Note down the resistance displayed on the multimeter (lower numbers means more absorption of EMF. Then add a table spoon of salt (kosher is ok, but sea or rock salt will give you a larger effect). You'll notice how resistivity--the technical term for resistance to EMF absorbtion--plummets.
We now proofed that the EMF absorbility of water is caused by salt, so we loose nothing by taking the water out of the equation.
In summary, the best way to prevent EMF exposure in hybrid cars is putting salt nest to the battery. Normal kitchen salt will probably do, but as shown above, rock salt is superior and also has other health effect. According to my calculations and simulations with Wolfram Research's excellent Mathematica software, about 2 pounds per Ampere hour is the optimal weight. For a 2006 Prius, this would be about 13 pounds of rock salt.
And additional benefit is that a rock salt crystal also improves mileage slightly by straightening out EMF lines and keeping them closer to the battery.
Since I published this article, I got a number of inquiries, so I will keep updating the FAQ:
Q: Won't rock salt in my trunk cause corrosion?
A: Good question. Most of the chassis of a Prius is made of aluminum which is not very susceptible to salt-induced corrosion. If you are concerned, you can pack the rock salt crystal into waxed paper (do NOT use zip lock bags because they are not biodegradable).
Q: Salt is hydroscopic, so won't the crystal melt?
A: There will be some loss of mass, but since the volume to surface area is very high in a 13 pound salt crystal, it is pretty negligible.
Q: Do I have to change the crystal regularly?r
A: Not really, EMF is fairly light, so the crystal can absorb a lot before loosing its effectiveness. If you are really concerned, weight the crystal occasionally and replace it when it shows a mass increase of more than 4 oz.
Q: What do I do with a used up crystal?
A: While EMF binding in salt is very stable, and it would take a lot of energy and specialist knowledge to extract EMF form a salt crystal, I understand that you don't want to throw a contaminated crystal into the trash or have it around your kids. Contact your local recycling authority. If your town doesn't have one, you can also usually drop it off at your power company who will extract the EMF and use it to generate electricity.