Introduction to Operator Exposure Risk Assessment for Pesticides

Little Pro on 2020-01-03 Views:  Update:2020-01-03

An operator exposure risk assessment is usually conducted to determine if the use of a pesticide product poses unacceptable health risks to pesticide operators and to support the registration of a pesticide product. In this article, we will give you a quick overview of the basic principles of the pesticide operator exposure assessment and show you how this is done.

Definition of Pesticide Operators

Pesticide operators mainly refer pesticide mixers/loaders and applicators.They are the main protection targets of the operator exposure risk assessment.

  • Mixer/Loaders: Individuals who mix a concentrated pesticide product with water and/or load it to the container of application equipment;
  • Applicators: Individuals who operate application equipment and apply a pesticide product onto its target.

Basic Principles of Pesticide Operator Exposure Assessment

Firstly, we determine an acceptable operator exposure level (AOEL) to pesticide operators based on animal toxicology studies. AOEL is defined as the maximum amount of active substance to which the operator may be exposed without any adverse health effects (Unit: mg of active substance per kilogram body weight of the operator, mg a.i./kg bw/d).

Secondly, we conduct exposure assessment and estimate daily operator exposure levels (unit: mg a.i./kg bw/d) based on pesticide application rate, application area, application equipment, and what personal protection equipment (PPE) or respiratory protection equipment (RPE) is used by operators. 

Finally, we compare estimated operator exposure levels with AOEL values and calculate risk quotients (RQ). If RQ<1 (meaning estimated exposure level lower than AOEL), we can conclude that the use is safe to pesticide operators.

  • Risk Quotient (RQ) = Estimated exposure level(mg a.i./kg bw/d)/AOEL(mg a.i./kg bw/d)
  • RQ<1: Risk acceptable.

Note: In the US, AOEL is not derived. Estimated exposure level is directly compared with relevant NOAEL value obtained from short-term animal toxicology studies.

  • Margin of Exposure (MOE) = Animal NOAEL (mg/kg bw/d)/Estimated total exposure (mg/kg bw/d);
  • MOE>100: risk acceptable. 

How to Derive Acceptable Operator Exposure Level (AOEL)

AOEL is usually derived by dividing the lowest NOAELs obtained from short term animal toxicology studies (i.e., 28d rat, 90d rat, 1-year dog) with uncertainty factors (Default UF=100). This is because operator exposure to pesticide active substance is usually intermittent over the intermediate-term. You can either derive AOEL for a specific exposure route (dermal or inhalation) or you can derive a ‘systemic’ AOEL value to cover all routes of exposure. It should be noted that 

  • AOEL = Relevant NOAELs/UF x Absorption correction factor

Note: Since an AOEL is usually expressed as an internal dose, the external NOAEL (applied dose) needs to be converted to an internal value by using a correction factor for systemic availability, especially if the absorbed dose at the NOAEL is significantly lower (in current practice <80 %) than the applied dose. 


Pesticde A, oral NOAEL (90d rat) = 1mg/kg bw/day, oral absorption rate obtained from toxicokinetic studies = 30%

  • AOEL = 1mg/kg bw/100 * 30% = 0.003 mg/kg bw

Pesticde B, oral NOAEL (1-year dog) = 1mg/kg bw/day. It is well absorbed. Oral absorption rate obtained from toxicokinetic studies = 85%

  • AOEL = 1mg/kg bw/100 = 0.01 mg/kg bw

Exposure Assessment: Estimating Operator Exposure Levels

Daily operator exposure levels can be estimated using the equation below.

  • Daily exposure dose = Unit exposure (mg ai/kg handled) × kg ai handled/day × % absorbed/Body weight
  • Kg ai handled/day = application rate (kg ai/ha) x treatment area (ha)

Unit exposure levels are expressed as mass of pesticide active ingredient exposure per unit mass of active ingredient handled (e.g.,mg/kg ai). Default unit exposure values are usually given/developed by authorities for different scenarios irrespective of chemical identity. A scenario refers to a specific type of application equipment, formulation type, job function, and level of personal protective equipment (PPE).

The most commonly used pesticide unit exposure data can be found in the following two documents:

For example, the US EPA unit exposure levels for mixer/ loader/applicator (assuming backpack sprayer, turf/plants/bushes/trees) depend on the level of level of personal protective equipment (PPE).

  • Dermal (single layer, no gloves): 58400 ug/lb, equivalent to 128.75 mg a.i./kg
  • Dermal (single layer, gloves): 30500 ug/lb, equivalent to 67.24 mg a.i./kg
  • Inhalation (no respirator): 69.1 ug/lb, equivalent to 0.152 mg a.i./kg
  • Inhalation (with PF10 respirator): 6.91 ug/lb, equivalent to 0.0152 mg a.i./kg

I have shown you how to find unit exposure data. We still need to obtain dermal and inhalation absorption data to calculate realistic operator exposure levels as the default dermal absorption rate of 100% is too conservative. For inhalation route, we always assume 100% for absorption. For dermal route, you can obtain estimated in vivo human absorption rate from in vitro and in vivo studies using the equation below. 

  • In vivo human absorption (%) = in vivo rat absorption (%) × in vitro human absorption (%) / in vitro rat absorption (%)

Now let's estimate daily operator exposure level for pesticide A (backpack sprayer, for plants, no gloves) based on the following assumptions:

  • Application rate: 0.075kg ai/ha (max proposed rate)
  • Treatement area: 1 ha/day (max treatment area by one person per day)
  • Absorption: 2% for dermal route, 100% for inhalation route
  • Body weight: 70kg
  • Unit exposure - dermal route: 128.75 mg a.i./kg
  • Unit exposure - inhalation route: 0.152 mg a.i./kg


  • Estimated exposure level - dermal route: 0.00276 mg a.i./kg
  • Estimated exposure level - inhalation route: 0.00016 mg a.i./kg
  • Estimated total exposure level (systemic): 0.00292 mg a.i./kg

Since estimated total exposure level 0.00292 mg a.i./kg is lower than the AOEL of pesticide A (0.003 mg a.i./kg), we can conclude that the operator risk of the  proposed use is acceptable. However, if the rate goes higher, the risk will become un-acceptable if operators do not wear gloves. Wearing gloves can mitigate the risks.

Note: Dermal absorption rates for un-diluted products and diluted products can vary significantly. Usually, dermal absorption rate for diluted product is higher. Using higher dermal absorption rate is recommended for pesticide operator exposure assessment.


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 Tags: Topics - CRAToxicology and Health Risk Assessment

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