Scientists have long debunked conspiracy theories about chemicals being secretly dispersed in the sky using aircraft.

Proponents of the theory variously allege that the chemicals are used to modify the weather or control people’s thoughts.

Despite the lack of evidence that they exist, Louisiana has nonetheless passed legislation banning the release of chemicals “for the express purpose of affecting the temperature, weather, climate, or intensity of sunlight”.

The bill cleared the Louisiana House of Representatives by 58-32 last Thursday, local TV channel WVUE reports.

Republican legislator Kimberly Landry Coates, defending the bill in the House, insisted that she regularly sees “chemtrails” in the sky and that they are released by “at least nine federal agencies”.

“When you look up in the air, you look, watch for the big white lines across the sky,” she said.

The white lines seen trailing behind aeroplanes are called contrails and are harmless trails of condensation.