2.1 Ventilation systems should be designed to minimise the intake of external air pollutants following paragraphs 2.2 to 2.9 if either of the following applies.

a. The pollutant values in the location of the dwelling exceed any of the limits in Table 2.1. This may have been determined through an air quality assessment. Where modelling or monitoring data is required, expert advice should be sought.

b. The dwelling is located near to any of the following sources of significant local pollution.

i. Road traffic, including traffic junctions and underground car parks.

ii. Combustion plant (such as heating appliances) running on conventional fuels, most commonly natural gas.

iii. Other combustion processes (for example, waste incineration, thermal oxidation abatement systems).

iv. Discharges from industrial processes.

v. Fugitive (i.e. not effectively controlled) discharges from industrial processes and other sources.

vi. Exhaust discharges from building ventilation systems.

vii. Construction and demolition sites, which are a source of particles and vaporous discharges.

viii. Other significant sources of local air pollution which may be detrimental to health.

Table 2.1 Limit values from Schedule 2 to the Air Quality Standards Regulations 2010

Control of ventilation intakes

2.2 Ventilation intakes should be located away from the direct impact of the sources of local pollution.

NOTE: CIBSE’s TM64 and TM40 give further guidance.

2.3 Where urban traffic is a source of pollution, the air intakes for dwellings next to busy urban roads should be both of the following.

a. As high as possible.

b. Located on the less polluted side of the building.

Mechanical ventilation may be the most practical way of achieving this requirement.

2.4 If practicable, ventilation intakes should not be located in courtyards or enclosed urban spaces where air pollutants are discharged. If this is unavoidable, intakes should be located to meet both of the following conditions.

a. As far as possible from the source of pollutants.

b. In an open or well-ventilated area.

2.5 In areas where wind often comes from opposing directions (e.g. a valley), the air intakes should point in the opposite direction to the exhaust outlets.

2.6 Where sources of pollution vary with the time of day, such as urban road traffic, it may be acceptable, for time-limited periods, to take one of the following actions.

a. Reduce the flow of external air into ventilation intakes.

b. Close ventilation intakes when the concentrations of external pollutants are highest.

NOTE: In these circumstances, expert advice should be sought.

Location of exhaust outlets

2.7 Exhaust outlets should be located so that both of the following are achieved.

a. Re-entry of exhaust air into a building, or entry into nearby buildings, is minimised.

b. There is no harmful effect on the surrounding area.

2.8 Where there is a prevailing wind direction, exhaust outlets should be downwind of intakes.

2.9 Exhaust outlets should not discharge into any of the following.

a. Courtyards.

b. Enclosures.

c. Architectural screens.

NOTE: Chapter 13 of McGraw Hill’s Indoor Air Quality Handbook provides further guidance.

Section 2: Minimising the ingress of external pollutants