Additive effects of radio-frequency radiation?

In the context of its effects of living beings, does RF transmission stack with the number of transmitters?

My utility company, PG&E, has been rolling out its wireless Smart Meters over the past few years.

There’s been intense debate about how safe they are, and the science seemed to suggest they were fine, but what happens when there are a bunch of them together?

I live in an apartment complex and the exterior wall of my bedroom has about 30 of them directly next to each other. How does that affect their transmission safety, if at all?

(I’m not sure how often, or how continuously, they transmit. But the basic question stands. Imagine a stadium with 10,000 cell phones if that’s any easier.)

There are lot of issues in this question, and it isn’t intuitive.

A pair of transmitters that are transmitting on exactly the same frequency and a given distance apart will result in a radiation pattern that has peaks and nulls in it due to constructive and destructive interference. This is similar to how you get high gain antennas. However because you don’t really know what the phase relationship between the two is, and because the frequencies are unlikely to be absolutely identical, the interference patterns will be unstable and you can only usefully talk about simple averaging of power. So, assuming the transmitters are close together relative to your distance to them, you will get double the power so long as both transmitters are transmitting. So if you had a slab of thirty transmitters all in a rack, and each one was operating at the same time you could measure an average power thirty times greater than if one were operating. However all transmitting together this isn’t going to happen in reality. Exactly what will or can happen gets complicated with modern transmission techniques, but given the very very low amount of information the smart meters need to transmit, and the short time they need to transmit it, the chance of even two wanting to overlap is close to zero. In principle, using spread spectrum, or TDMA mechanisms, you could have two or more working together. But the critical issue is that bandwidth is expensive. All those smart metres compete for limited bandwidth, bandwidth that costs real money to use. There is absolutely no useful reason to have them operate longer than is needed and good reasons for them not to.

The question of a stadium full of cell phones becomes more interesting. At a distance large relative to the size of the stadium, the power is again simply additive. So if you were a mile away, the power you saw at a mile for all phones active would be 10,000 times the power you saw a mile away from just one phone active. Either way, a very small amount of power. Near the stadium and a distance of a similar scale to the size of the stadium and closer, you enter the near field, and something interesting happens. The power density remains constant. If you imagine you are in crowd, the people next to you all have phones, so say 8 people with a metre. So 8 times the power of one person within a metre. then there are say 16 people within two metres. But the power from each of those people is down to 1/4 that of the people close by, so the total power is from them only an additional four times. Then the next layer out is 24 people, but the power is now 1/9 from each one, so a bit over 2 times added. Then 32 people and 1/16 the power, and so on: 40/25, 48/36, 56/49, 64/64, 62/81, and as each new circle of people is added around you, although the are more and more people in each layer, the contribution of power they add is less and less, the entire circle of 64 people 8 away from you adds the equivalent power of one person next to you. If you had an infinitude of people the power would become infinite, but you don’t. What basically happens is that the power you see in a arbitrarily large crowd is that the power contributed by the crowd is about the same as the power you see from the cell phone in your pocket. So at worst it doubles.

Can you point us to this science? You’d need to look at what assumptions went into it. Did they examine the case where there are 30 together, or did they just look at one. Did they assume people would typically be 10 meters or more away, or did they consider a bedroom with the meters just on the other side of a wall?

30 of them, however they work, is going to be 30 times the power compared to just 1. That’s a significant factor, but maybe they included that possibility in their assessment.

ETA: They make foil wallpaper. Foil wallpaper on that wall would go a long way towards shielding you from the RF. If you raise a stink, you could probably get PG&E to pay for it.

Maybe if it were made from copper. In any event, we’ve been bombarded by RF for the last 80 years.

I’m not aware of any smart meters that transmit more than 1 watt on the 900MHz band. That’s puny. Nor am I aware of any smart meters that transmit continuous duty, generally they are polled at some interval and the more meters that exist, the slower the poll rate will be. As you can imagine, less transmitting means less RF exposure.

I’d be more worried about holding a cell phone up to my face for 20 minutes at a time. 500 milliwatts at 1/4" inch is going to radiate more power in your body than 1 watt at several tens of feet, attenuated by stucco, framing, wallboard, and whatever else is between your domicile and the meters.

In any event, to answer the OP directly, not only are RF exposure limits based on averaged exposure time, they’re also segregated between occupational exposure and general population exposure. The devices of which you speak are generally covered by the Part 15 of the FCC’s rules, and are not dissimilar to cordless phones, baby monitors and other devices that make use of the ISM band.

Perhaps I’ve missed it, have there been any causative reports of 900MHz cordless cell phones causing cancer and the like in the last 20 years? No doubt the general population has received much more RF from these devices, simply because they’ve been in existence much longer and are used in a manner which exposes the user to more RF than a smart meter does.

To be clear: TV stations have engineers that will spend 10-12 hours a day at RF dense sites (such as Mt Wilson near Los Angeles, one of those most RF dense locations in the world) and nobody is aware of any causative link between the mega amounts of RF these folks are exposed to day in and day out and any ill health effects.

If anyone is aware of such empirical data, please share it, I’m confident folks in the industry would read it with eagerness.

Don’t get me wrong-- RF exposure can be bad. Touch an element on an antenna putting out 50kW and you’re going to get burned. Put your nuts in front of a microwave transmitter and you’re probably saving on birth control for the rest of your life.

Aluminum foil is plenty conductive enough. It’s not like copper is the only magical conducting metal. If it conducts well enough to be shiny in visible, it’s conducting well enough to reflect RF.

Power has nothing to do with whether electromagnetic radiation is harmful or not (unless it is enough to cause burns); only once you get to ultraviolet does the individual photon energy get high enough to cause chemical reactions and DNA damage; visible light should be far more dangerous than radio waves but nobody has ever claimed that visible light causes cancer or other harm (not counting studies of the effects of nighttime lights and such, which are due to other biological effects). And yes, visible light can be as effective as microwaves (or near-microwave radiation) in cooking.

How do you figure that? It would make no sense at all that any hazards from RF would be independent of the exposure.

The only way that radio-frequency radiation can ever hurt any living thing is via heating, and we’ve evolved pretty good mechanisms to prevent that. If you feel the smart meters burning you, go “ouch”, and recoil from them, then they’re damaging you. If you don’t, they aren’t. This constitutes the entirety of the scientific “debate” about RF: There is no other side, scientifically speaking. The panicmongers who oppose this fact have no science on their side.

As I said, power is only relevant if you can get burns from it (or maybe, to put it a better way, thermal effects, since you will die from hyperthermia before you get actual burns); otherwise, it doesn’t matter how much power is being produced. Even with a hundred (thousand?) cellphones in the same room, you aren’t going to absorb enough power to feel it, unless perhaps they were placed all over your body (and the heat from the phones themselves would probably be a bigger issue, since the total power used by a cellphone is several times the RF output).

By contrast, when I talk of energy, I am referring to the energy of the individual waves, which increases with frequency and is what can cause chemical damage (heat by itself can, but only if it gets too hot). Here is a chart of frequency and energy; microwaves (specifically, UHF, 300 MHz-3 GHz, as most-used in communications) have about 1.24-12.4 millionths of an electron volt (ev) of energy while visible light is about 1.65-3.3 ev (equivalent to 400 THz for red to 800 Thz for violet).

Another way to explain this: solar cells only respond to photons if their energy is equal to or greater than the band-gap of the semiconductor used; thus for silicon, photons with less than 1.12 ev of energy might as well not exist, in much the same way most chemical compounds don’t “respond” to energies lower than that corresponding to UV.

This talk about the energy of a photon is irrelevant. PG&E should be able to point to their source that 30 of the meters they are using, at the frequency they are transmitting, are safe. And something more scientific than “Well you can’t feel it, so it must be safe”.

If their source says 20 microWatts, for the frequencies they’re using, 30 minutes a day is safe, and the average exposure from a single meter is 2 microWatts, 30 minutes a day, then they haven’t shown that Reply is safe, because he’s getting the equivalent of 60 microWatts, 30 minutes a day.

Conversely, if they’ve shown 200 microWatts, 60 minutes a day is safe, and the meters radiate 0.1 microWatts, 10 minutes a day, then Reply has nothing to worry about. Really, with all the research that has gone into this, this shouldn’t be hard for PG&E to do.

The difference here is ionizing vs. non-ionizing radiation.

When most people hear the word “radiation” these days, what they are really thinking of is ionizing radiation, like x-rays and gamma rays (and Godzilla). Radio waves, microwaves, infra-red, visible light, ultra-violet, x-rays, gamma rays, etc. are all just different frequencies of electromagnetic radiation. Part way through the ultraviolet part of the spectrum though it becomes high enough in frequency that it can strip the electrons off of atoms and create ions. This is why it is called “ionizing” radiation. UV, x-rays, and gamma rays are all ionizing. Lower frequency radiation like visible light, IR, microwaves, and radio waves are non-ionizing.

Ionizing radiation is bad because it causes cell damage, cancers, and other bad things. With non-ionizing radiation, you only get damage when you get enough of it to cause heat damage. The inside of a microwave oven can cook you, but so can a large enough spotlight with visible light. With ionizing radiation, you get damage at significantly lower power levels. Ionizing radiation will kill you long before you feel any heat from it. There’s a big difference between ionizing and non-ionizing radiation.

Radio frequency radiation is lower in frequency than visible light, and therefore is (as far as we know) just as harmless as visible light. Nobody worries about shining a flashlight on your skin, but everyone panics if there is a tiny low power radio transmitter somewhere.

The OP’s smart meters are probably equivalent to 1 watt light bulbs (your typical living room lamp is going to be more like 40 watts), so if you picture tiny 1 watt flashlights flashing periodically that’s about how dangerous they are.

Again, this talk about ionizing radiation has bupkis to do with Reply’s question.

Reply, if you download the RF Summary Discussion (PDF), found here, they give levels for Immediately adjacent to a SmartMeterTM device (1 foot) of 8.8 μW/cm^2.

They also state that the FCC limit is 601 μW/cm^2 for the 902-928 MHz band they operate at. (ETA: that may be a communication interference limit, not a health limit, but at least it’s something.) You can’t be 1 foot away from all 30 at once, but if you were, you’re average exposure would be 264 μW/cm^2. That’s a factor of 2 below the limit.

(If they were all operating at the same frequency, they could add coherently at certain locations to higher levels (while canceling at others), but I don’t think they could possibly all transmit at the same frequency and still work, so adding power is appropriate.)

It wouldn’t hurt to call PG&E, and say that you’re concerned because there are so many. See what they say. It’s possible they’re all going to be tied to a single transmitter, and then you’d be back down to the 8.8 μW/cm^2. At least you’d sleep better.

I went looking to see if the FCC limit was based on communications or health (it was based on health), but the report I linked to above may have glossed over the issue.

From this page, there’s a link to a Final Report (5 MB PDF) on health impacts from the Smart Meters. On page 18 (PDF page 20), it says (spaces didn’t show up in copy/paste)

I would have read that to mean 8.8 μW/cm2 while it’s transmitting, for short periods of time. From the text of the following, however, that 8.8 μW/cm2 is the average power over a duty cycle. The power when actually on is 360 μW/cm2, over half the FCC limit. It would only take two transmitting simultaneously to push you over the 601 μW/cm2.

It also says that the FCC allows averaging over 30 minutes, so just those two wouldn’t be an issue, but if they go for 4 minutes, followed by seven other pairs, you’d be over the FCC limit.

I’ve got to go to dinner now, but Reply, I’d again suggest calling PG&E, and expressing concern about the bank of 30 meters. Hopefully they’ve only got one transmitter for all 30 of them.

Get them to pay for some foil hats while you’re at it. It will help protect against, among other things, aliens.

The legal limit is 601 μW/cm^2 at the frequency of the Smart Meter. Are you implying that limit should just be ignored?

No, I’m implying that worry about EMF in this case is needless and paranoid.

The study measures these power densities at one foot distance. If you re-read my posting above about the stadium full of cell phones you will note that you can’t get close enough to enough meters to allow the power density to rise greatly. Read the top of page 24 for the reality of what to expect.

Comparded to using a cell phone or household cordless phone smart meters vanish. And phones are vanishingly trivial comapared to many other things.

Yes, I acknowledged that in post 13.

Sure. Do you want that before or after the study they release that says the power poles they use aren’t going to come to life and stab you up? I mean, they must have dozens of those bad boys out there.

Worrying about RF radiation is pointless for the reasons people have given above: Unless radiation is ionizing, which RF radiation is not by definition, it can only hurt you if it burns you in the same way a hot stove would burn you. This is freshman-level physics, either high school or (more commonly) college, and having to prove it to the satisfaction of every nutter out there lining their wall with aluminum foil isn’t worth anyone’s time.