COSHH Assessment Calculator

The Control of Substances Hazardous to Health Regulations 2002 require employers to assess and control exposure to hazardous substances at work. This includes dusts, fumes, vapours, gases, and biological agents. A COSHH assessment must be carried out before work begins.

WELs are the maximum concentration of a substance in the air that workers can be exposed to over an 8-hour period (LTEL) or 15-minute period (STEL). They are published in HSE EH40/2005 (Table 1). Common construction WELs (8h TWA): respirable crystalline silica 0.1 mg/m³, hardwood dust 3 mg/m³ (softwood 5 mg/m³), Portland cement 4 mg/m³ respirable / 10 mg/m³ inhalable, isocyanates 0.02 mg/m³ (with a 0.07 mg/m³ 15-min STEL). Asbestos has a separate control limit of 0.1 fibres/ml over 4 hours under CAR 2012.

HSE Construction Information Sheet CIS36 (Revision 3) puts the general level for construction dust at an APF of 20. Its Table 2 lists that as an FFP3 disposable mask or a half mask with a P3 filter, and for longer duration work "Powered RPE such as a TH2 powered hood/helmet". Note the class: a powered respirator is not automatically APF 40. HSG53 (Fourth edition) Figures 13 and 14 give powered devices three protection factors, TH1 and TM1 at 10, TH2 and TM2 at 20, TH3 and TM3 at 40, and the class is marked on the device. FFP2 (APF 10) is only adequate for low-level general nuisance dust. All tight-fitting RPE must be face-fit tested and the wearer clean-shaven at the seal; loose-fitting powered hoods and helmets (TH, BS EN 12941) do not need a fit test, but powered masks (TM, BS EN 12942) do.

Asbestos is not assessed by a simple exposure-divided-by-APF check. Under the Control of Asbestos Regulations 2012 most asbestos work is licensed or notifiable and must be enclosed, air-monitored and clearance-tested. The control limit is 0.1 fibres/ml averaged over 4 hours, and regulation 11(5) is the duty not to exceed it. A separate figure of 0.6 fibres/ml over 10 minutes is often quoted alongside it, but the two are not the same thing: 0.6 f/ml is the approved concentration for "sporadic and of low intensity" exposure under regulation 2(4), which decides whether work is licensable, and ACOP L143 paragraph 28 states expressly that it is not the control limit. The HSE minimum RPE is APF 20, an FFP3 disposable or half-mask P3. ACOP L143 paragraph 308 requires power-assisted full-facepiece P3 (APF 40) or better for licensable work inside enclosures, and allows FFP3 or half-mask P3 for low-risk ancillary tasks outside the enclosure such as scaffold erection, site set-up, enclosure dismantling and waste handling, so the driver is expected exposure rather than the licence category. FFP2 is never acceptable for asbestos. If you may disturb asbestos, stop and engage an HSE-licensed contractor.

APF indicates how much a respirator reduces exposure. APF 10 means it reduces exposure by 10x, APF 20 by 20x, APF 40 by 40x. The protected exposure = actual exposure / APF. Choose RPE where the protected exposure is below the WEL. The APF belongs to the device class, not to the broad type of device: HSG53 (Fourth edition) Figure 13 gives powered respirators with masks TM1 10, TM2 20 and TM3 40, and Figure 14 gives powered hoods and helmets TH1 10, TH2 20 and TH3 40. Read the class marking on your own equipment rather than assuming a powered unit is the highest class, because assuming 40 where the device is a TH2 halves the exposure you think you are breathing.

Technically, COSHH applies to employers, but the duty to manage your own health exists under the Health and Safety at Work Act. In practice, principal contractors require COSHH assessments from all workers on site. Many accreditation schemes also require them.