Some facts about GENERAL VENTILATION & LEV as taken directly from HSE web site;-

General ventilation

All workplaces need an adequate supply of fresh air. This can be natural ventilation, from doors, windows etc or controlled, where air is supplied and/or removed by a powered fan. If you work in an office or shop, natural ventilation will normally be enough to control dusts and vapours from cleaning materials etc. Sometimes planned, powered general ventilation is an integral part of a set of control measures, eg the welding of large fabrications in a workshop.

Local exhaust ventilation

Local exhaust ventilation (LEV), or extraction, is an engineering control solution to reduce exposures to dust, mist, fume, vapour or gas in a workplace. Use a properly designed LEV system that will draw dust, fume, gases or vapour through a hood or booth away from the worker. An extraction system should be easy for workers to use and enclose the process as much as possible. It should effectively capture and contain the harmful substance before it is released into the working environment. Air should be filtered and discharged to a safe place. The system should be robust enough to withstand the process and work environment. It is important to maintain it and undertake tests to ensure it is working effectively.

Things to avoid when applying LEV and Common errors in applying extraction are:

the effectiveness of small hoods is usually overestimated – be realistic
the hood is usually too far away from the process
the hood doesn’t surround the process enough
inadequate airflow
failure to check that the extraction continues to work
workers are not consulted, so they don’t understand the importance of extraction and do not use it properly