Someone did a study at MIT about tin foil hats, and found that not only do they not screen radio interference, in some cases, can actually magnify them.

Conclusion: The helmets amplify frequency bands that coincide with those allocated to the US government between 1.2 Ghz and 1.4 Ghz. According to the FCC, These bands are supposedly reserved for ‘‘radio location’’ (ie, GPS), and other communications with satellites (see, for example, [3]). The 2.6 Ghz band coincides with mobile phone technology. Though not affiliated by government, these bands are at the hands of multinational corporations. It requires no stretch of the imagination to conclude that the current helmet craze is likely to have been propagated by the Government, possibly with the involvement of the FCC. We hope this report will encourage the paranoid community to develop improved helmet designs to avoid falling prey to these shortcomings.