To block a wave you have to short it. Provide something conductive of a suitable length. 1 mm of ali will short a mm MW and is much easier to do than 150m radio wave would be. Waves can be klms in the ELFs.
The problem is biology shorts them wanted or not. Melanin in the skin is a shorting conductive layer. If they get through skin and short deep inside thats not good news obviously. Wave lengths the scales of human anatomy are bad news as they do get shorted setting up alien charges inside biology. This are MWs and shorter. We have evolved in light so are resistant to it. Longer waves dont short so are not so bad.
We short these waves cos we are made of salted (metal oxide) water. If we were not salt and conductive it would not be a problem.