Using and interpreting data from the National Pollutant Release Inventory


The NPRI is a key tool for identifying and monitoring pollution sources in Canada and can be a starting point for better understanding how pollutants are entering the Canadian environment.

This inventory collects information yearly on facility-based releases to air, water and land as well as disposals and transfers of over 300 substances and substance groups. The data is reported on an annual basis by facilities that meet the NPRI reporting requirements.

The general NPRI reporting requirements are designed to capture the most significant point-sources and relate to the quantity of the specific substance(s) manufactured, processed, used, or released at the facility during the year; the total number of hours worked by employees at the facility; and/or the type of activities performed at the facility. For more information on these reporting requirements, see the NPRI Reporting Tools and Guidance.

The NPRI makes a significant amount of data available to data users, and this data can be analyzed in a number of ways. For example, it can be analyzed by substance, by facility, by media (air, water, land), by geographic region, by industrial classification code, and/or by type of release to the environment (direct releases, disposals, transfers). NPRI data can also be combined with other data sources, such as monitoring data collected under other programs, to provide a more comprehensive picture of pollution in various areas of the Canadian environment.

There are considerations however, to be taken into account by data users to ensure the data is analyzed, combined and interpreted properly. The purpose of this guide is to describe these considerations.

1.0 General Considerations for Analyzing NPRI Data

1.1 The NPRI does not cover all sources of pollution in Canada

  • The NPRI includes information on a select list of over 300 substances. The NPRI Substances List was originally created in 1993 using the US Toxics Release Inventory (TRI) list as a starting point but adapted for Canadian circumstances. The NPRI Substances List is reviewed on an ongoing basis using the latest available information. Since 1993, there have been a number of changes to the list via additions, deletions, or changes to reporting thresholds. Reasons for why some substances of concern are not included in the NPRI include, for example: facility-based releases are not deemed an important source of exposure in Canada, the substance is tracked under another program1 , or because inclusion on the NPRI has not yet been considered (e.g. emerging priorities, etc.). Changes to the NPRI substance list can be proposed by any person, government or organization - see the Consultations page for more details.
  • Some smaller facilities are exempt from reporting, as are facilities undertaking certain activities or in certain sectors. The NPRI is designed to focus on significant facility-based (point source) pollutant sources, meaning that certain small facilities or facilities carrying out certain activities are not captured by the NPRI. Non-point (e.g. diffuse, mobile, etc.) pollution sources are also not captured by the NPRI, nor are pollutants arriving in Canada from other countries (i.e. transboundary pollution).

1.2 NPRI Pollutants are not all equally hazardous

  • NPRI substances all have distinct properties and range in toxicity. For example, a release of a small amount of a highly toxic substance may be of more concern than a larger release of a less toxic substance. Therefore, in order to understand release data, and/or combine release data from different substances, data users must acknowledge the variability among substance properties.

1.3 Assessment of risks cannot be determined from NPRI data alone

  • Characterization of risks to the environment and human health is complex and context dependent. As noted above, substances on the NPRI are not all equally hazardous, and evaluation of impacts requires contextual information. Factors to consider include (among others) a substance’s inherent toxicity; physical and chemical properties (e.g. persistence, bioaccumulation, etc.); the medium to which it is released (e.g. to air, land or water); transport and transformation pathways (i.e. if and to what extent it is broken down or transported in the environment); and the resulting amount, timing, nature and level of exposure. Data users are encouraged to consider relevant contextual information to avoid misinterpreting perceived risk from NPRI data alone. Environment and Climate Change Canada and Health Canada continue to assess the health and environmental risks of chemical substances through the Chemicals Management Plan and other programs.

1.4 Calculation methods used for quantifying releases can vary

  • A variety of methods may be used by facilities to quantify or estimate release and transfer data, including continuous emission monitoring systems, predictive emission monitoring, source testing, mass balance, site-specific emission factor, published emission factor, or engineering estimates. This flexibility is in place to accommodate reporting of information on releases, disposals and transfers that the facility owner/operator possesses, or may reasonably be expected to have access to. The limitation, however, is that accuracy and uncertainty of these calculations may vary according to the method used. For more information see the NPRI Toolbox.

1.5 Efforts to improve NPRI data quality are in place

  • Despite best efforts by facilities, errors sometimes occur in data reported to Environment and Climate Change Canada. Environment and Climate Change Canada implements a number of measures under the NPRI Data Quality Framework to ensure the quality of NPRI data (relevance, accuracy, reliability, completeness, understandability, accessibility, and timeliness) in order to continue to meet the needs of data users. Please consult the data quality page for more information on how Environment and Climate Change Canada works to address and improve NPRI data quality.

    As part of data quality control activities, Environment and Climate Change Canada reviews data reported by facilities for inaccuracies and reporting errors, and contacts facilities as necessary to verify and update their reports. The most common errors are the use of inappropriate reporting units and decimal errors (such as reporting in kilograms instead of tonnes). Other types of errors include: omitting a substance or source of release; using incorrect reference data or assumptions for calculations (e.g., using quantities of all substances instead of only NPRI substances; reporting quantities manufactured, processed or otherwise used instead of quantities released); providing erroneous geographic information (facility location); or erroneous or inconsistent North American Industry Classification System (NAICS) codes.

1.6 NPRI data is not a measure of compliance with regulatory or other requirements

  • NPRI data is not an indication of whether a facility is in compliance with regulatory or other requirements. In addition to being required to report to the NPRI, some reporting facilities may be subject to other regulatory or other standards under federal statutes as well as applicable provincial/municipal regulatory or other requirements. NPRI data does not directly indicate whether a facility is in compliance with these requirements, but may be a useful starting point for analyses of this type.

2.0 Factors Affecting Data Analysis and Comparability

2.1 Year to Year Analysis

Facilities may report different amounts of pollutants year-to-year and data users are encouraged to consider these when using NPRI data. Factors contributing to this variability can stem from:

  • Changes to NPRI reporting requirements: NPRI reporting requirements have evolved over time to provide more comprehensive information on pollutants in Canada. For example, substances have been added/removed, reporting thresholds have been reduced, and exemptions for several industry sectors have been eliminated. When conducting year-to-year trend analysis, please consult the history of consultations and the list of changes to the NPRI over time in order to account for any reporting requirement changes that may impact the year-to-year comparability. Figure 1 illustrates how the history of changes to NPRI reporting requirements have affected the year-to-year comparability by altering the number of facilities and/or substances reported.
  • Figure 1: Changes in reporting to the NPRI, 1993-2014.

  • Figure 1 illustrates how the history of changes to NPRI reporting requirements have affected the year-to-year comparability by altering the number of facilities and/or substances reported.
  • Long Description

    Linear graph with reporting year as the x-axis, number of substances on the NPRI as the left-hand y-axis (0-400), and the number of reporting facilities on the right-hand y-axis (0-10,000). The graph shows two broken data lines; one that tracks the number of substances added to the NPRI and a second that tracks the number of facilities reporting to the NPRI between 1993 and 2014. It shows an increase in the number of facilities reporting that correlates with the addition of substances to the NPRI list. There were 1,388 reporting facilities in the first year, in 1993, whereas there were 7,720 in 2014. Several changes in the list of NPRI-reportable substances account for this increase. The NPRI was set up in 1993 with 178 substances. In 1999, 73 substances were added. In 2000, 17 polycyclic aromatic hydrocarbons, dioxins and furans, and hexachlorobenzene were added. In 2002, seven criteria air contaminants were added, almost doubling the number of facilities required to report. In 2003, 60 differentiated volatile organic compounds were added. In 2007, total reduced sulphur, nine polycyclic aromatic hydrocarbons and 17 dioxins and furans were added.

  • Changes at the facility level: Year-to-year comparability may be affected by a number of possible changes at the facility level, including changes in production levels, facility expansions/reduction, temporary closures, process modifications, pollution prevention plan implementation, accidental releases, materials or feedstock substitution, product design or reformulation, equipment or process modifications, spill and leak prevention, on-site reuse, recycling or recovery, improved inventory management or purchasing techniques, improved operating practices or training, modifications, procedures or practices other than pollution prevention activities.
  • Changes in calculation method: A variety of methods to calculate releases and transfers may be used by facilities reporting to the NPRI and a facility may change their method(s) used from year-to-year.
  • Data updates: Facilities may from time to time update the data they previously reported for any given year. These updates are submitted in the online reporting system, but may not be made publicly available until the following reporting cycle.
  • Changes in the regulatory environment (risk management measures): In addition to reporting releases to the NPRI, reporting facilities must comply with applicable environmental initiatives or standards under federal statutes such as the Canadian Environmental Protection Act, 1999, and the Fisheries Act as well as other applicable provincial/municipal measures. These risk management actions can help explain the changes in NPRI data over time and data users are encouraged to consider these during NPRI data interpretation. For example, environmental performance agreements between Environment and Climate Change Canada and certain facilities have led reductions in polycyclic aromatic hydrocarbon releases since their implementation. Further general details can be found on Canada’s approach on chemicals substances, and the Chemicals Management Plan, and specific substance approaches can be found linked from the List of Toxic Substances - Schedule 1.

2.2 Substance to Substance Analysis

  • Simple addition of reported quantities of different substances may not provide a meaningful measure of total pollution. It may be misleading to directly compare or add NPRI data for different substances, as not all substances on the NPRI are equally hazardous (i.e., all have distinct properties and levels of toxicity). As noted above, in order to understand release data, and/or combine release data from various different substances, data users must acknowledge the variability among substance properties and how these affect the context of (potential) hazard.
  • Comparison of substances may be affected by units of measurement: Quantities may have been reported using different reporting units (i.e. kilograms versus tonnes) so users are encouraged to be mindful of units when making comparisons.

2.3 Facility to Facility Analysis

  • Calculation methods may vary from facility to facility, even for the same substance, and accuracy may vary according to the calculation method used. These methods may also change from year to year even at the same facility. Data users are encouraged to consider the method used by a facility for calculating releases when examining and comparing NPRI data.

2.4 Comparisons between Pollutant Release and Transfer Registers (PRTR)

  • Data in the NPRI may not be directly comparable to the PRTR data from other programs such as the U.S. Toxics Release Inventory (TRI). The various programs have different reporting requirements (e.g., thresholds, exclusions, substance lists, etc.). Consequently, quantities of substances reported to the NPRI and other PRTRs may be different even for very similar facilities. Data users are encouraged to review each PRTRs reporting requirements when conducting comparisons. For more information on efforts to improve the comparability of PRTRs, please consult the Commission for Environmental Cooperation (CEC)’s Action Plan to Enhance the Comparability of Pollutant Release and Transfer Registers (PRTRs) in North America .

2.5 Avoiding Double-counting

Adding NPRI data reported by different facilities, or under different parts of the NPRI requirements may result in double-counting or other errors in interpretation. Some potential scenarios to be aware of include:

  • Total reduced sulphur consists of 6 substances. Three of these substances (hydrogen sulphide [H2S], carbon disulphide [CS2] and carbonyl sulfide [COS]) are also listed individually in the NPRI substance list. When conducting analysis of NPRI data, caution should be taken to avoid "double counting" of total reduced sulphur and these individually-listed substances. See figure 2 for example
  • Figure 2: Example of potential for double-counting using total reduced sulphur data.

  • Figure 2 gives an example of incorrect (e.g. double-counted) total reduced sulphur calculations, and an example of the correct total reduced sulphur calculation.
  • Long Description

    Figure 2 is a picture of a spreadsheet, and shows two different addition columns; the left one demonstrate an incorrect (double-counted) way to count total sulphur releases (TRS) and the right one demonstrates the correct way. TRS is a group containing 6 sulphur compounds, three of which are also listed individually on the NPRI list. The correct count on the right shows the TRS total on the bottom row (15,523 tonnes) as a sum of the individual quantities of the 6 compounds contained in the TRS group in rows above. That is: carbonyl sulphide (5,159 tonnes), carbon disulphide (4,458 tonnes), hydrogen sulphide (2,707 tonnes), and “other (remaining 3) TRS compounds” (3,199 tonnes). By contrast, the incorrect way shows a larger TRS total on the bottom row (27,847 tonnes), as a sum of the TRS total (15,523 tonnes) AND the individually-listed sulphur compound quantities (carbonyl sulphide (5,159 tonnes), carbon disulphide (4,458 tonnes), hydrogen sulphide (2,707 tonnes) in rows above.

  • Particulate Matter (PM) data:There are different reporting thresholds for particulate matter, i.e., PM 2.5 (0.3 Tonne threshold) < PM10 (0.5 Tonne threshold) < total PM (TPM) (20 Tonne threshold). As such, a facility may meet the threshold(s) for one or two of the smaller fractions because these are lower, but not necessarily meet the threshold for the larger fraction because it is significantly higher. Also, as per the figure below, the TPM fraction includes PM10 and PM 2.5, while PM10includes PM 2.5. Caution is recommended when manipulating this data to both to avoid double-counting, and to consider the threshold level when looking at each PM fraction. Also, data users should note that PM data may also contain quantities of substances (i.e. some metals) reported as releases to air under other parts of the NPRI, and caution is advised to avoid double-counting.
  • Figure 3 demonstrates the relationship between the three particulate matter fractions using concentric circles.
  • Figure 3: Relationship between Particulate Matter size fractions

    Long Description

    Figure 3 demonstrates the relationship between the three particulate matter fractions. Concentric circles show the PM2.5 fraction (particles less than or equal to 2.5 micrometers in diameter) as the smallest circle contained inside the medium-sized PM10 fraction circle (particles less than or equal to 10 micrometers in diameter), contained inside the larger-sized TPM fraction circle (particles equal to or greater than 100 micrometers).

  • Polyaromatic Hydrocarbons (PAHs): Part 2 of the NRPI reporting requirements cover a list of PAH species for which facilities can report on individually, or as “PAHs, total unspeciated” (if information on individual PAHs is not available or they are in small quantities), or as a combination of speciated and unspeciated PAHs. When conducting analysis of NPRI data, caution should be taken to avoid "double counting" of speciated and unspeciated PAHs.
  • Volatile Organic Compounds (VOCs): Total VOCs are reportable as a group under Part 4 of the NPRI reporting requirements; however a number of individual VOCs (about 100) are also reportable under Part 1A of the NPRI reporting requirements. Similarly, 20 VOCs are subject to further (more detailed) reporting under Part 5. Care should be taken to avoid double- or triple-counting of the same VOCs reportable to multiple NPRI parts.
  • Substances reported as Transferred for disposal: In some cases, adding quantities of a substance transferred off-site (e.g. to specialized waste management facilities) to quantities of that substance disposed of (e.g. by the waste management facility) can lead to double-counting. For example, this may occur when a substance is generated at one site, then transferred to another site for waste treatment prior to disposal, and subsequently transferred to a final disposal site. In this case, double- or triple-counting of the original off-site transfer may occur if aggregating reports from the several facilities involved in the waste management process of a single quantity of substance.

2.6 Releases versus Transfers and Disposals

  • A release to air, water or land represents the direct entry of that substance into the environment, whereas disposals/transfers don’t necessarily do so. It is important to note that substances reported as disposals to a regulated landfill, tailings impoundment, waste rock management area or other disposal area do not necessarily represent the same direct entry into the ambient environment as releases to air, water, or land. For example, a substance reported as a disposal in waste rock may indicate that it is contained naturally in low concentrations in rock removed during mine operations and disposed of in a permitted area for long term storage and management, not entering the ambient environment directly.

2.7 Tailings and Waste Rock Data

The following are important considerations for the use, analysis and interpretation of National Pollutant Release Inventory (NPRI) data on tailings and waste rock (TWR):

  • Spike in tailings and waste rock reports between 2005 and 2006: The difference in the amounts of NPRI substances reported for 2005 as compared to 2006 and more recent years are mainly due to changes in NPRI reporting requirements related to substances contained in tailings and waste rock. See figure 1 above. These are more fully explained on the webpage: Historical Proposals and Consultations for Changes to the NPRI.
  • Tailings and Waste Rock concentration data:Facilities are required to report the concentration of NPRI-listed substances in tailings and waste rock. They also have the option of providing additional comments about the specific substances they report on. This information is available by searching for an individual facility on the NPRI Online Data Search, and then viewing the report for a specific substance.
  • Impact of geology on reporting: Differences in geology explain many of the different values reported by similar facilities. The reported quantities of NPRI substances in tailings and waste rock generally reflect naturally-occurring substances in the rock or bitumen deposits removed and/or processed by mining facilities. As a result, differences in the quantities of substances reported for disposal by facilities may reflect differences in geology and in the type of mineral deposit. For example, consider two facilities with similar production levels and environmental management practices. A facility located in an area where the rock contains a naturally high level of arsenic would be expected to report larger amounts of that substance for disposal, compared with a facility in an area with lower levels of the substance.
  • Certain exclusions apply: Certain materials are exempt from reporting to the NPRI for tailings and waste rock. These include inert waste rock, sand in tailings, and materials used as structural components, for example, to build roads and dams. For more information on exclusions, please refer to the Guide for the Reporting of Tailings and Waste Rock to the NPRI.
  • Facilities may report negative values: for substances contained in waste rock or tailings. The disposal of mined materials in a waste rock or tailings management area is not necessarily a final disposal. For example, if market prices increase for a given metal or mineral, it may be profitable for a mining operation to “mine” or process materials previously disposed of as waste rock or tailings. Reporting of a "negative number" for waste rock or tailings indicates that the quantity of a substance removed from the management area exceeded the quantity of the substance deposited in that area for a given year.
  • Tailings and Waste Rock management: A number of government and industry initiatives are in place to manage potential environmental concerns from tailings and waste rock. Specific initiatives include the Metal Mining Effluent Regulations, the Environmental Code of Practice for Metal Mines, the Mine Environment Neutral Drainage Program and the National Orphaned/Abandoned Mines Initiative. For more information on these and other initiatives, please see the NPRI related links page.


The NPRI provides a significant amount of data that, if used properly, can be a useful starting point for better understanding how pollutants are entering the Canadian environment. The considerations presented here are provided to help data users analyze, combine and interpret NRPI data properly. For more information about the program and/or using and interpreting NPRI data, please see the webpage on Frequently Asked Questions about the NPRI,or contact the NPRI office by E-mail:

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