The concept of ‘critical loads’ for soils has been developed as a reference for policies to reduce acid deposition as a result eg of emissions from power stations (see Section 6.10). A ‘critical load’ is the amount of a substance that may be deposited in a given area over a given timescale without adverse effects. It will depend on the character of the soil and so will vary from area to area. It can be regarded as a special kind of environmental quality standard. The concept of critical loads became widely accepted in the late 1980s as a means to assess pollution control options for acid deposition. It was extensively promoted by Sweden and taken up enthusiastically by the UK. It formed the basis for national emission limits for acidifying substances under the UNECE Convention on Long-Range Transboundary Air Pollution (see Section 13.2) and in the proposed EU national emission ceilings Directive (see ‘Proposals being developed’).