For toxicologists and risk assessors accustomed to evaluating chemical safety through dose-threshold comparisons (exposure vs. DNEL/DMEL), the USEtox approach to human toxicity assessment can seem fundamentally problematic. The core issue centers on the use of DALY (Disability-Adjusted Life Years)—a well-established epidemiological metric for quantifying real health burden—to express what is actually a comparative lifecycle index.

This terminological choice creates systematic confusion that undermines communication between LCA practitioners and risk assessment professionals and obscures the distinction between comparative sustainability metrics and actual safety assessment.

Before examining the specific problems with how USEtox uses DALY, it's important to acknowledge that the USEtox documentation is technically clear about the model's comparative nature. However, as explored in the companion article on ecotoxicity terminology (PAF/PDF confusion), technical clarity in documentation does not prevent systematic misunderstanding when the chosen terminology directly contradicts its established meaning in the professional community.

For human toxicity, this problem is particularly acute because:

• DALY has one unambiguous meaning in public health and toxicology—years of healthy life lost due to a specific disease
• Risk assessors expect threshold-based evaluation, not linear summation across all lifecycle chemicals
• The terminology suggests quantification of real health burden when the metric is designed for comparative ranking

This article examines how the DALY terminology creates confusion, what the conversion factors behind USEtox scores actually represent, and why terminological reform is essential for productive collaboration between LCA and toxicology communities.

What DALY Means in Public Health and Toxicology

In public health, epidemiology, and toxicology, DALY (Disability-Adjusted Life Year) is a well-established metric for quantifying disease burden:

DALY in public health:

DALY = YLL + YLD

Where:

• YLL = Years of Life Lost (due to premature death)
• YLD = Years Lived with Disability (weighted by severity)

Key characteristics of public health DALY:

• Population-based measurement: Calculated from actual health statistics (mortality data, disease incidence, disability prevalence)
• Observable burden: Represents real health impacts that can be verified through epidemiological studies
• Disease-specific: Calculated separately for each disease or condition (e.g., DALYs from malaria, DALYs from diabetes)
• Used for resource allocation: Guides public health priorities and interventions based on actual disease burden

Example of public health DALY:

The Global Burden of Disease Study 2019 estimates that cardiovascular disease caused approximately 393 million DALYs (95% uncertainty interval: 368-417 million) globally in 2019. This means:

• Real people died prematurely (YLL component)
• Real people lived with heart disease, angina, heart failure (YLD component)
• The metric quantifies actual health burden experienced by populations

Interpretation: "393 million years of healthy life were lost globally due to cardiovascular disease in 2019"

This is a measurement of real health impacts at the population level, based on observed mortality and morbidity data.

For toxicologists and risk assessors, DALY represents:

• Real health outcomes from chemical exposures that exceed safe thresholds
• Measurable disease incidence in exposed populations
• Quantifiable burden that can be prevented through exposure control
• Epidemiological evidence linking exposure to health effects

When a toxicologist sees "DALY," the interpretation is unambiguous: actual years of healthy life lost in real populations.

What "DALY" Means in USEtox and other LCA models

In contrast, the "DALY" appearing in USEtox characterization factors represents something conceptually different:

USEtox characterization factor for human toxicity:

CF [CTUh/kg] = FF [days] × iF [dimensionless] × EF [cases/kg intake]

Result unit: [CTUh/kg emitted] = [cases/kg emitted]

Then converted to DALY:

DALY/kg emitted = cases/kg emitted × DALY/case

What this represents:

The USEtox "DALY" is a calculated comparative index based on:

• Persistence (FF): How long chemical mass remains in environment
• Intake fraction (iF): What fraction of emitted mass is eventually ingested/inhaled by human population
• Effect factor (EF): Cases of disease per kg human intake (no threshold evaluation)
• Severity factor: DALY burden per disease case (e.g., 11.5 DALY/cancer case)

The critical differences from public health DALY:

• No actual exposure measured: Based on hypothetical 1 kg emission to standardized continental/urban compartment
• No threshold assessment: Does not evaluate whether exposures would actually exceed safe levels (DNEL/DMEL)
• No population specificity: Uses generic continental or urban population model
• Sums all chemicals: Assumes population is exposed to ALL chemicals in product lifecycle
• Linear dose-response: Every exposure contributes, regardless of magnitude relative to safe thresholds

For toxicologists and risk assessors, this is problematic because:

• It doesn't distinguish between safe and unsafe exposures
• It sums contributions from different populations at different lifecycle stages
• It expresses a comparative index using health burden terminology
• It cannot answer the fundamental question: "Is this safe?"

Behind the "DALY" Conversion: Simplifications and Uncertainties

What Most Practitioners Don't Know

When USEtox converts "cases" to "DALY" by multiplying by 11.5 (cancer) or 2.7 (non-cancer), most practitioners and clients assume these are well-established scientific constants. In reality, these are statistical averages from 1990 epidemiological data with significant simplifications and enormous uncertainty ranges.

Understanding what lies behind these conversion factors is essential for proper interpretation of USEtox scores—and reveals why the term "DALY" is particularly misleading in this context.

The Conversion Mechanism

The actual conversion is straightforward:

DALY endpoint [DALY/kg emitted] = CTUh midpoint [cases/kg] × Severity Factor [DALY/case]

• For cancer: Severity Factor = 11.5 DALY/case
• For non-cancer: Severity Factor = 2.7 DALY/case

Source of these factors: Huijbregts et al. (2005), "Human-toxicological effect and damage factors of carcinogenic and noncarcinogenic chemicals for life cycle impact assessment" (Integrated Environmental Assessment and Management, 1(3):181-244)

Methodology: "Incidence-weighted DALY" from the Global Burden of Disease (GBD) Study 1990

What the Single Cancer Factor (11.5 DALY) Masks

The factor 11.5 DALY/case represents a weighted average across ALL cancer types from GBD 1990 data. This was a practical choice given the constraints of 2005: most chemicals are known to cause "cancer" without specification of which type, and implementing 20+ cancer-specific factors would have been impractical.

However, this simplification masks substantial heterogeneity: Cancer severity varies dramatically - from very lethal cancers like pancreas (9% survival) to highly curable cancers like prostate (98% survival). USEtox applies a single severity factor (11.5 DALY) to all types, creating systematic bias when comparing substances that cause different cancer types.

Recognition by the Scientific Community That Updates Are Needed

The scientific community has recognized these limitations and recommended updates—though these have not yet been implemented in USEtox.

CE Delft (2021) assessment:

"This conversion is based on data from 2005 (Huijbregts et al., 2005) which is considered outdated and lacks differentiation between distinct health conditions. Some estimates for disease burden expressed in DALY were already proposed for different types of cancers (Zhou et al., 2015)."

Fantke et al. (2021)—USEtox Taskforce update recommendations:

"Severity factors [...] were originally based on incidence-weighted DALY from the Global Burden of Disease (GBD) study series for 1990 [...] We recommend using incidence-weighted DALY values for all cancer endpoints from Huijbregts et al. (2005)... [and] that DALY values per incidence for both non-cancer categories be updated with the most recent GBD statistics (Salomon et al. 2015; Stanaway et al. 2018)."

Fundamental question: What is the scientific value of the resulting score?

(my) Honest answer:

✅ Valid for qualitative comparisons between very different alternatives (factor >10× difference)

❌ Not valid for quantifying real health burden (it's a comparative index using hypothetical emissions)

❌ Not valid for fine optimization between similar options (difference smaller than uncertainty)

❌ Not valid for regulatory decisions (no relationship to DNEL/safety thresholds)

The Core Problem: Terminology, Not Methodology

The USEtox methodology, understood correctly, is a reasonable comparative tool for its intended purpose. The primary problem is terminological:

Using "DALY" (Disability-Adjusted Life Years)—an established epidemiological metric for real health burden—to express a comparative index creates systematic misinterpretation.

Most users (LCA practitioners, industrial clients, policymakers, even many researchers) see "15.3 DALY" and interpret:

❌ "This product causes 15.3 years of life lost in the population"

❌ "We can expect 15.3 years of disability from this exposure"

❌ "15.3 years of healthy life will be lost if this product is used"

What "15.3 DALY" actually means in USEtox:

✅ "A hypothetical 1 kg emission to a standardized continental compartment, summing all exposed populations across lifecycle stages, without threshold evaluation, yields a comparative persistence-intake-toxicity index equivalent to the severity-weighted disease incidence from epidemiological averaging, with 1-3 order of magnitude uncertainty"

How USEtox Definitions Imply Real Health Measurement

What the USEtox Documentation Actually Says About Human Toxicity

It's important to document exactly what the official USEtox documentation states about the interpretation of human toxicity scores.

At Midpoint level:

"The characterization factor for human toxicity impacts at midpoint level (human toxicity potential) is expressed in comparative toxic units (CTUh), providing the estimated increase in morbidity in the total human population per unit mass of a contaminant emitted, assuming equal weighting between cancer and non-cancer effects due to a lack of more precise insights into this issue."

Elsewhere, the documentation states:

"The midpoint human toxicity potential [disease cases] or [CTUh] per kg chemical emitted can be interpreted as the increase in population risk of a morbidity effect due to an emission in a compartment."

And:

"The effect factor EFhum,ef,xr [disease cases/kgintake] can be interpreted as the increase in the number of cases of a given morbidity (e.g. cancer or non-cancer diseases) risk [dimensionless] in the exposed population per unit mass ingested or inhaled [kgintake]."

At Endpoint level (damage):

"The damage (endpoint) level human toxicity potential [DALY] or comparative damage unit [CDUh] per kg chemical emitted represents an increase in adversely affected life years as a consequence of an emission in a compartment."

And:

"The area of protection in LCA that is relevant for human toxicity is Human Health, and damage to this area of protection is typically expressed in LCA as a loss of healthy life years."

These Definitions Imply Real Health Measurement

Despite including "Comparative" in its unit names (CTUh, CDUh), USEtox documentation describes results using language that systematically implies actual health effects rather than comparative indices:

Official definitions state:

• "Increase in morbidity in the total human population" (not "comparative morbidity index")
• "Increase in the number of cases" (not "case-equivalents for comparison")
• "Increase in adversely affected life years" (not "life year index")
• "Loss of healthy life years" (not "comparative health burden metric")

Every key term—increase, morbidity, cases, loss, affected life years—describes real, observable health outcomes in epidemiology and public health. None are qualified as "hypothetical," "for comparative purposes," or "index values."

The result: A practitioner reading these definitions will naturally conclude that USEtox measures actual disease cases and real loss of life years—not because they misunderstand LCA methodology, but because the text as written describes real health effects.

The word "Comparative" appears only in technical acronyms that non-experts often overlook, while all descriptive language uses unqualified epidemiological terminology. This linguistic structure makes misinterpretation inevitable, not a user education problem.

Recommendations: Toward Honest Nomenclature

To reduce systematic misinterpretation while preserving USEtox's valid comparative purpose, terminological reform is essential:

Neutral Comparative Index Units

Replace:

• CTUh [cases/kg emitted]
• CDUh [DALY/kg emitted]

With:

• Human Toxicity Potential Index [HTPᵢ]
• Human Toxicity Damage Index [HTDᵢ]

Units: Dimensionless or arbitrary units (e.g., "index points" or "toxicity-persistence units")

Advantage: Immediately clear that this is a comparative ranking metric, not a quantification of real health impacts

Conclusion: Honest Tools, Misleading Language

To be absolutely clear: USEtox human toxicity assessment is methodologically sound for its intended purpose—comparative ranking of product alternatives in LCA. The developers have been scientifically rigorous in their approach and transparent in their technical documentation. The problem is not the science; it's the terminology.

1. Professional skepticism between LCA and toxicology communities—similar to the PAF confusion in ecotoxicity assessment
2. Communication failures when presenting results to clients, regulators, or decision-makers
3. Undermined credibility of LCA methods when users discover scores don't mean what the terminology suggests
4. Barriers to Safe & Sustainable by Design implementation that requires productive collaboration between both assessment approaches

The fundamental problem: You cannot use the language of health impact assessment (DALY, cases, morbidity, loss of life years) while delivering a comparative ranking tool, and expect systematic correct interpretation.

This terminological issue must be addressed—either through nomenclature reform or, at minimum, through prominent disclaimers that ensure every user understands what USEtox scores actually represent versus what the terminology appears to claim.