The smoke could be seen from Garston, Watford Town Centre, Borehamwood, Bricket Wood, Bushey, Chorleywood and St Albans.
At around 1pm the fire in Oakridge Lane and Kemprowwas tweeted, but as of yet no actual start time, or if casualties.
Roads closed off by police and 3 fire engines on scene - The Car Recovery business yard was well alight. Firemen said their was fuel and possibly gas bottles.
The smell of burning rubber and plastic filled the air along with regular small bangs.
A police spokeswoman said: "Police were called by the fire service at 1.59pm to reports of a large fire in Aldenham.
An investigation to the cause of the fire is on going.
more footage and story to follow.
Herts Fire Control said on Twitter: "Further to our previous tweets regarding the SCRAPYARD FIRE in ALDENHAM - if you are in close proximity to the area of Aldenham and Radlett - please ensure your windows and doors are closed to avoid inhaling the smoke fumes."
View or smoke plume— 999 Herts News 🚨 (@999herts) September 29, 2018
from watford town pic.twitter.com/ySAIisNVHI
Helping supply water to extinguish 2000 tons of scrap metal well alight in Aldenham this afternoon/evening/night!! The smoke could be seen across London apparently! @HVPCapability @HertsFRSControl @JDSemps #weneededabiggerpump pic.twitter.com/80WMNFwN0a— Welwyn Garden City Green Watch (@WGCGreenWatch) September 29, 2018