For Radon gas there is guidance this in a published report by BRE BR211, ‘Radon: Guidance on protective measures for new dwellings 1999’
Ceir cyfarwyddyd ynghylch nwy radon mewn adroddiad a gyhoeddwyd gan y Sefydliad Ymchwil Adeiladu - BR211, 'Radon: Guidance on protective measures for new dwellings 1999'.