When running a commercial kitchen there is a standard to which you need to keep your kitchen held to, so that you avoid hazards, including dangers to the general public and your employees. So a company was set up dedicated to upholding kitchen standards and protecting the general public, they decided to make this easier by providing a book with guidelines to be followed to regulate these standards, this book is known as the TR19.
Over time your extraction fans will also build up a dust and grease deposits which can be very harmful to your entire kitchen, not only will the ducting be effected, but your entire establishment due to the air not being extracted.