As expected, exposure varies based on agricultural activity and the types of crops grown. Potential for exposure to 2,4-D is much higher in areas where cereal crops, potatoes, and fruit are more commonly grown due to the higher application rates for these crops. The prairie provinces (Alberta, Saskatchewan, Manitoba), Prince Edward Island and Southern Quebec all had higher potential for exposure to 2,4-D than other areas. These estimates primarily consider exposure through inhalation, which is a more immediate form of exposure, but pesticides can also seep into ground water and soil and lead to subsequent exposure. The CAREX Canada estimates do not account for dietary ingestion, which is a major route of pesticide exposure, especially for young children.[6]

In addition to those living near agricultural areas, the spouses and children of pesticide applicators are more likely to be exposed (termed para-occupational exposure) compared to the general population.[7] This is because 2,4-D may be brought into homes on the bottom of shoes, increasing potential for exposure especially in young children through dermal means and hand to mouth transfer.[8] This is an important exposure route for young children because they spend the majority of their time indoors, and because 2,4-D persists longer indoors than outdoors.[8] In children younger than 3 years of age, non-dietary exposure to 2,4-D accounted for 15% of total exposure.[9]

In the Canadian prairies, 2,4-D is the most frequently detected herbicide in rainfall; it is present in up to 93% of samples collected.[10] 2,4-D may enter the atmosphere when it volatilizes during and after spray applications, when drops of 2,4-D are transported by wind currents after spraying, and when 2,4-D adheres to dust and is incorporated in clouds during the cloud forming process.[11]

Data sources

Actual pesticide application rates were not available for Canada, so pesticide usage was estimated based on crop types grown and typical application rates for these crops in North America. Data were obtained on the distribution and acreage of different crop types, 2,4-D application rates, and the number of people living in each area. Data on agricultural crops were obtained from Agriculture and Agri-Food Canada’s Annual Crop Inventory for 2016, which are digital images (raster datasets) that produce an average crop accuracy of 89%.

Data on pesticide usage came from the United States Department of Agriculture’s Agricultural Chemical Use Program and the Government of Ontario’s Ministry of Agricultural, Food and Rural Affairs. These datasets summarize how much 2,4-D was applied to crops along with the average yearly application rates. Group averages were applied if specific crop data were not available. For instance, we averaged the 2,4-D application rates for pears, peaches, and apples to estimate the application rate for “orchard fruits”, which was a crop type captured by the Annual Crop Inventory. Average annual 2,4-D application rates for all crops were obtained.

Population data from Canada’s 2016 Census of Population were used to estimate the number of people potentially exposed. Population estimates are based on the number of people living within the census subdivision (CSD).

Methods and analysis

Estimates were calculated at the CSD level (n=5,054). Specifically, we calculated the total area (in km²) of land within each CSD for each crop type. Total areas were then multiplied by the average 2,4-D application rate data for each crop. Finally, the total was summed together to obtain an estimate of 2,4-D use in kilograms for each CSD. Results were weighted in square kilometres to illustrate the kilograms of 2,4-D applied per square kilometre in each CSD. CSDs with less than 0.2 kg/km² were classified as having no exposure to account for very small agricultural activity within the CSD. Data are presented in quartiles and integrated with population data to show potential exposure and variations across the country.