Y ddau brif lygrydd aer sy'n peri'r pryder mwyaf yw nitrogen deuocsid a gronynnau. Allyrrir y ddau wrth i ddiwydiant a cherbydau ymlosgi tanwydd, a daw gronynnau o lwch y ffordd a thraul breciau a theiars hefyd. Prif ffynonellau’r llygryddion hyn mewn ardaloedd trefol yw dulliau trafnidiaeth. Mae'r sector trafnidiaeth eisoes wedi gwneud cynnydd i leihau allyriadau ac mae'n parhau i wneud hynny, yn bennaf drwy reoliadau Ewropeaidd ynghylch allyriadau cerbydau. Mae tystiolaeth sy'n dod i'r amlwg yn nodi bod rheolaethau ar allyriadau ar gyfer cerbydau diesel wedi methu â chyrraedd y lleihad disgwyliedig.
The two main air pollutants of concern are nitrogen dioxide and particulate matter. Both are emitted by the combustion of fuel by industry and vehicles, and particulate matter also comes from road dust and brake and tyre wear. The main sources of these pollutants in urban areas are transport. The transport sector has made and continues to make advances in reducing emissions, largely through European vehicle emission regulations. Emerging evidence indicates that emissions controls for diesel vehicles have fallen some way short of the reductions anticipated.