Wednesday, August 13, 2008

Wind Turbines and Epilepsy ... Not

One of the stranger accusations sometimes leveled at wind projects is that the flickering shadows from wind turbines' rotating blades can trigger seizures in epileptic individuals. Here and here are useful resources that refute this myth.

More generally, shadow flicker is a very well-known phenomenon that occurs seasonally when the sun is very low in the sky (casting longer shadows). Determining the number of hours in a year that a property neighboring a wind farm will experience flicker, and exactly where, is a straightforward calculation. Noble Environmental Power, a wind developer, has developed an excellent fact sheet on shadow flicker. Here is what it says on epilepsy:
Is there a connection between shadow flicker and epilepsy?

No. Shadow flicker from wind turbines cannot trigger epileptic seizures in individuals suffering from photosensitivity, as some opponents have claimed. The frequency, or the number of times something happens per second, is measured in Hertz (Hz). Shadow flicker from wind turbines has a frequency between 0.5 Hz and 1.25 Hz, which is equivalent to between 1 to approximately 1.25 alternations per second. This is well below the range of frequencies that can trigger epileptic seizures, which is 5 to 30 Hz, according to the American Epilepsy Foundation. Thus, shadowing from wind turbines poses absolutely no threat to the health of people with epilepsy or other individuals who are photosensitive.



Charles Barton said...

Wind industry propaganda hides the truth again.

See, "Wind turbines, flicker, and photosensitive epilepsy: Characterizing the flashing that may precipitate seizures and optimizing guidelines to prevent them," by Graham Harding, Pamela Harding, and Arnold Wilkins.


[quote]Wind turbines are known to produce shadow flicker by interruption of sunlight by the turbine blades. Known parameters of the seizure provoking effect of flicker, i.e., contrast, frequency, mark-space ratio, retinal area stimulated and percentage of visual cortex involved were applied to wind turbine features. The proportion of patients affected by viewing wind turbines expressed as distance in multiples of the hub height of the turbine showed that seizure risk does not decrease significantly until the distance exceeds 100 times the hub height.

Since risk does not diminish with viewing distance, flash frequency is therefore the critical factor and should be kept to a maximum of three per second, i.e., sixty revolutions per minute for a three-bladed turbine. On wind farms the shadows cast by one turbine on another should not be viewable by the public if the cumulative flash rate exceeds three per second. Turbine blades should not be reflective. [/quote]

Tom Gray said...

And this is typical of wind opponents, who don't much care about facts as long as they can present a negative view of wind.

"Flash frequency ... should be kept to a maximum of ... 60 revolutions per minute for a three-bladed turbine."

A typical wind farm turbine rotates at 17 or 18 rpm. As I said, the frequency of the flicker is well below the range that affects photosensitive individuals.