Industrial Bleaching Chemistry and Infant Exposure Science: A Technical Review of ECF and TCF Processes, Dioxin Formation Potential, and Safety Implications for Chlorine-Free Diapers

This scientific analysis explains the real-world differences between Elemental Chlorine Free (ECF) and Totally Chlorine Free (TCF) pulp bleaching, clarifies how dioxins form in industrial processes, and—most importantly—addresses whether either process has any impact on baby health, safety, or skin exposure. The conclusion is clear: the ECF vs TCF distinction is an environmental manufacturing issue, not a baby safety issue.

The Science of Dioxin Formation in Pulp Bleaching 

Dioxins are formed as unintentional by-products of several industrial processes involving chlorine, including waste incineration, chemical manufacturing, and traditional pulp bleaching. In older elemental chlorine bleaching systems, molecular chlorine reacts with lignin and other organic compounds in wood pulp, forming chlorinated organic by-products, including dioxins and furans. Because these compounds are environmentally persistent and capable of bioaccumulation, the pulp and paper industry began moving away from elemental chlorine bleaching in the 1990s.

Chlorine and Hypochlorite as Bleaching Reagents

During bleaching, chlorine substitutes hydrogen atoms on the aromatic rings of lignin through electrophilic aromatic substitution. It also oxidizes lignin side-chain groups to carboxylic acids and adds across carbon–carbon double bonds in lignin side chains. While chlorine can also react with cellulose, this reaction occurs primarily near neutral pH, where un-ionized hypochlorous acid (HOCl) is the dominant chlorine species in solution.¹ To minimize excessive cellulose degradation, chlorination is therefore carried out at strongly acidic conditions (pH < 1.5).

Cl2 + H2O ⇌ H+ + Cl− + HClO

At alkaline pH values (> 8), the dominant species becomes hypochlorite (ClO⁻), which is also effective for lignin removal. Sodium hypochlorite may be purchased directly or generated in situ by reacting chlorine with sodium hydroxide:

2 NaOH + Cl2  ⇌ NaOCl + NaCl + H2O

The primary environmental concern associated with chlorine-based bleaching is the formation of large quantities of soluble organochlorine compounds (commonly measured as AOX), which are released into surrounding water systems. These compounds include trace levels of dioxins and furans that are environmentally persistent and toxic at extremely low concentrations.

So, what is the real chemistry of how dioxins form during chlorine bleaching?

There is no single, clean step chemical equation where HOCl directly turns into dioxin.
Dioxins form through a multi-step reaction pathway involving:

• Phenolic lignin fragments

• Hypochlorous acid (HOCl)

• Oxidation + radical coupling

• High-energy conditions in bleaching

But for reference, I can show you the accepted mechanistic pathway in clear chemical stages, which is what many scientific papers actually discuss.

Mechanism of Dioxin Formation from Hypochlorous Acid

Molecular chlorine hydrolyzes in water to form hypochlorous acid (HOCl), the primary chlorinating species in pulp bleaching:¹

Cl₂ + H₂O ⇌ HOCl + H⁺ + Cl⁻

Hypochlorous acid reacts with lignin-derived phenolic structures to form chlorophenols, the direct chemical precursors to dioxins:²–⁴

C6H5OH + HOCl → C6H4ClOH + H₂O

Further chlorination produces polychlorophenols:²,³

C6H4ClOH + HOCl → C6H3Cl2OH + H₂O

Under acidic, oxidative bleaching conditions, two chlorinated phenolic molecules undergo oxidative coupling, forming polychlorinated dibenzo-p-dioxins (PCDDs):⁵,⁶

2C6H3Cl2OH + [O] → C12H6Cl4O₂ (dioxin) + 2H₂O

Critical Scientific Takeaway:

✔ HOCl does NOT create dioxins on its own
✔ Dioxins only form if ALL THREE of the noted components exist together:

1. Free chlorine chemistry (HOCl)

2. Phenolic lignin fragments

3. Strong oxidative coupling conditions

✔ This is exactly why modern ECF/TCF bleaching suppresses dioxins to near-zero
✔ And why NO dioxins form in finished diapers!

What Is ECF Bleaching?

Elemental Chlorine Free (ECF) bleaching replaces elemental chlorine gas with chlorine dioxide. Chlorine dioxide is far more selective for lignin removal and produces dramatically fewer chlorinated organic by-products. Today, approximately 95% of the world’s bleached kraft pulp is produced using ECF technology, making it the global hygiene and medical-grade pulp standard.

Chlorine dioxide

Chlorine dioxide, ClO2 is an unstable gas with moderate solubility in water. It is usually 

generated in an aqueous solution and used immediately because it decomposes and is explosive in higher concentrations. It is produced by reacting sodium chlorate with a reducing agent like sulfur dioxide.

2NaClO3 + H2SO4 + SO2 → 2ClO2 + 2NaHSO4

Chlorine dioxide is sometimes used in combination with chlorine, but it is used alone in ECF (elemental chlorine-free) bleaching sequences. It is used at moderately acidic pH (3.5 to 6). The use of chlorine dioxide minimizes the amount of organochlorine compounds produced.8

Chlorine dioxide (ECF technology) currently is the most important bleaching method worldwide. About 95% of all bleached Kraft pulp is made using chlorine dioxide in ECF bleaching sequences.9

Why Modern ECF Bleaching Chemistry Does Not Generate Dioxins

Elemental Chlorine Free (ECF) bleaching uses chlorine dioxide (ClO₂) instead of elemental chlorine gas (Cl₂). This switch fundamentally changes the chemistry of how lignin is removed and, in practice, shuts down the classic dioxin-formation pathway that depends on HOCl + chlorophenols + oxidative coupling.

1. No HOCl Pool → No Chlorophenol Pathway

In classical elemental-chlorine bleaching, Cl₂ hydrolyzes to hypochlorous acid (HOCl), which is the strong electrophile that chlorinates phenolic lignin rings to chlorophenols—the immediate precursors of dioxins:

Cl₂ + H₂O ⇌ HOCl + H⁺ + Cl⁻

Ar–OH (phenolic lignin) + HOCl → Ar–Cl (chlorophenol) + H₂O

2Ar–Cl + [O] → polychlorinated dibenzo-p-dioxin (PCDD) + H₂O

In a pure ECF system, there is no elemental chlorine stage, so you do not get this HOCl equilibrium. Chlorine dioxide behaves differently in water:

2ClO₂ + H₂O → HClO₂ + HClO₃     (chlorous + chloric acids)

Neither chlorous acid (HClO₂) nor chloric acid (HClO₃) is in equilibrium with HOCl the way Cl₂ is. Instead, they act primarily as strong oxidants rather than electrophilic ring-chlorinating agents.23 

Bottom line: The classical “HOCl → chlorophenols → dioxins” route that you described for elemental chlorine simply does not exist in a properly operated ECF line.

2. ClO₂ Is a Selective Oxidant, Not a Strong Aromatic Chlorinator

Chlorine dioxide reacts with lignin mainly by one-electron oxidation of phenolic units, producing phenoxy radicals that go on to quinones, ring-opened products, and carboxylic acids—not polychlorinated phenols. 22 A simplified representation is:

Ar–OH + ClO₂ → ArO• + H⁺ + ClO₂⁻      (single-electron oxidation)

ArO• + O₂ / further [O] → Ar=O (quinone) or ring-opened acids

Detailed mechanistic and kinetic studies of ClO₂ with lignin and lignin model compounds show:

• Phenolic lignin is preferentially oxidized (not multiply chlorinated).

• Chlorination is minor and mostly mono-substitution, often on side chains, not the dense ortho/para substitution pattern that leads to PCDD precursors.21

Gellerstedt and co-workers, along with later work by Liu and others, show that under typical ECF D₀ conditions (pH ~3–5, 50–70 °C), ClO₂ rapidly oxidizes lignin while strongly suppressing aromatic ring chlorination compared to Cl₂/HOCl bleaching. 21

AOX (adsorbable organic halogens) formed in ClO₂ stages is mainly a mix of low-molecular-weight chlorinated aromatics and ring-opened products; polychlorinated phenols are only minor components, and PCDD/PCDFs are not detected in these mechanistic model studies.23,24

3. ECF Process Design Starves the System of Dioxin Precursors

Modern ECF sequences (e.g., O–O–D₀–E/O or E/P–DₙD) are deliberately engineered to minimize both lignin and any residual free chlorine–type species at each stage:

• Low kappa number before bleaching: extended cooking + oxygen delignification remove most lignin before the first ClO₂ stage. Less lignin = fewer potential phenolic precursors. 25

• Moderately acidic pH (3.5–6) in D₀: optimized for selective oxidation of residual lignin; not a favorable environment for HOCl-type chemistry. 23 

• No NaOCl / hypochlorite stages in true ECF: hypochlorite is another classic source of chlorophenols and dioxins; it is explicitly removed in modern ECF designs. 25

Kinetic studies of AOX formation in the D₀ stage show that AOX formation is driven overwhelmingly by lignin content and ClO₂ dosage, not by HOCl or hypochlorite, and that even at high ClO₂ charge the AOX levels are far below those from historical Cl₂ / hypochlorite bleaching. 23

4. Real-World Data: Dioxins in ECF Mills Are at or Below Detection

The mechanistic story is backed up by a huge amount of field data:

• A Japanese kraft mill operating an extended-cooking, two-stage oxygen delignification + ClO₂ ECF line reported total dioxins in process sewers below 1 pg TEQ/L, and 2,3,7,8-TCDD was not detected in any ECF stage filtrate. 25

• The same study shows that converting from a conventional Cl₂/hypochlorite sequence to ECF cut 2,3,7,8-TCDD from 0.8 pg/L in the alkaline stage to ≤0.06 pg/L (below quantitation), and 2,3,7,8-TCDF similarly dropped to sub-pg/L levels. 25

• UNEP’s Stockholm Convention toolkit summarizes NCASI data from 20 ECF bleach lines: with complete chlorine dioxide substitution, 2,3,7,8-TCDD was not detected in any effluent sample above a 10 pg/L guideline; 2,3,7,8-TCDF was detected only rarely at very low pg/L levels. 26 

• For products, UNEP states that replacing Cl₂ with ClO₂ in bleaching reduces 2,3,7,8-TCDD and 2,3,7,8-TCDF in pulp to non-detectable levels, with modern ECF pulps in the range of 0.1–0.3 pg/g at most. 26

Where very low levels of other PCDDs appear in ECF mill waters, detailed congener analyses often trace them to external sources (e.g., legacy herbicides in river water, contaminated wood preservatives), not to in-situ formation by ClO₂ bleaching chemistry itself. 25

5. The Only Remaining Route Is Trace Cl₂ Contamination — and That’s Controlled

UNEP explicitly notes that any residual PCDD/PCDF formation in nominally ECF mills is due to small amounts of Cl₂ impurity in the ClO₂ solution combined with unusually high lignin (high kappa) entering the D₀ stage. When Cl₂ contamination is reduced to very low levels, the formation of 2,3,7,8-substituted PCDD/PCDF is driven down to background.23

Modern ClO₂ generators are designed to keep Cl₂ contamination extremely low, and mills monitor both PCDD/PCDF in effluent and AOX routinely. Combined with low-kappa operation and oxygen delignification, this means there is no practical pathway left for significant dioxin formation from the ECF chemistry itself.

ECF Chlorine Bleaching Summary:

• Classical dioxin formation requires HOCl + phenolic lignin + harsh oxidative coupling.

• ECF replaces Cl₂/HOCl with ClO₂, which acts as a selective oxidant, not a strong aromatic chlorinator.

• Process design (low kappa, oxygen delignification, no hypochlorite) removes lignin and HOCl-type species from the system.

• Large field datasets show non-detectable 2,3,7,8-TCDD/TCDF in ECF filtrates and pulps; any trace PCDD/PCDF is attributed to Cl₂ contamination or outside sources, not ClO₂ itself.

So in realistic, modern operation, ECF chemistry cannot meaningfully create dioxins—which is exactly why regulators and UNEP treat ECF as an Accepted Modern Technology with dioxin releases at or near background levels.

What Is TCF Bleaching?

Totally Chlorine Free (TCF) bleaching eliminates all chlorine-based chemicals and instead uses oxygen, hydrogen peroxide, and ozone to remove residual lignin. This further reduces the formation of chlorinated by-products in mill wastewater and is primarily adopted in a small number of environmentally focused markets.

Hydrogen peroxide

Using hydrogen peroxide to delignify chemical pulp requires more vigorous conditions than for brightening mechanical pulp. Both pH and temperature are higher when treating chemical pulp. The chemistry is very similar to that involved in oxygen delignification, in terms of the radical species involved and the products produced.10 Hydrogen peroxide is sometimes used with oxygen in the same bleaching stage and this is give the letter designation Op in bleaching sequences. Metal ions, particularly manganese catalyze the decomposition of hydrogen peroxide, so some improvement in the efficiency of peroxide bleaching can be achieved if the metal levels are controlled.11

Ozone 

Ozone is a very powerful oxidizing agent and the biggest challenge in using it to bleach wood pulp is to get sufficient selectivity so that the desirable cellulose is not degraded. Ozone reacts with the carbon-carbon double bonds in lignin, including those within aromatic rings. In the 1990s ozone was touted as good reagent to allow pulp to be bleached without any chlorine-containing chemicals (totally chlorine-free, TCF). The emphasis has changed and ozone is seen as an adjunct to chlorine dioxide in bleaching sequences not using any elemental chlorine (elemental chlorine-free, ECF). Over twenty-five pulp mills worldwide have installed equipment to generate and use ozone.12

Environmental Impact vs Product Safety

The true distinction between ECF and TCF exists at the wastewater discharge level, not in the finished diaper. TCF reduces industrial effluent chlorinated compounds slightly more than ECF. However, modern ECF mills already achieve extremely low emission levels that approach background environmental concentrations. After biological treatment and environmental dilution, both ECF and TCF effluents demonstrate very low measurable toxicity.

Conventional bleaching using elemental chlorine produces and releases into the environment large amounts of chlorinated organic compounds, including chlorinated dioxins.13  Dioxins are recognized as a persistent environmental pollutant, regulated internationally by the Stockholm Convention on Persistent Organic Pollutants.

Dioxins are highly toxic, and health effects on humans include reproductive, developmental, immune and hormonal problems. They are known to be carcinogenic. Over 90% of human exposure is through food, primarily meat, dairy, fish and shellfish, as dioxins accumulate in the food chain in the fatty tissue of animals.14

As a result, from the 1990s onwards, the use of elemental chlorine in the delignification process was substantially reduced and replaced with ECF (Elemental Chlorine Free) and TCF (Totally Chlorine Free) bleaching processes. In 2005, elemental chlorine was used in 19-20% of kraft pulp production globally, down from over 90% in 1990. 75% of kraft pulp used ECF, with the remaining 5-6% using TCF.15  Most TCF pulp is produced in Sweden and Finland for sale in Germany, all markets with a high level of environmental awareness.9  In 1999, TCF pulp represented 25% of the European market.16

TCF bleaching, by removing chlorine from the process, reduces chlorinated organic compounds to background levels in pulp mill effluent.17  ECF bleaching can substantially reduce but not fully eliminate chlorinated organic compounds from effluent. While modern ECF plants can achieve chlorinated organic compounds (AOX) emissions of less than 0.05 kg per ton of pulp produced, most do not achieve this level of emissions. Within the EU, the average chlorinated organic compound emissions for ECF plants is 0.15 kg per ton.18 To put this into perspective, 0.05kg/ton is equivalent to 0.05 ppm, which is the same as 5 cents of 1 million dollars.

However, there has been disagreement about the comparative environmental effects of ECF and TCF bleaching. Some researchers found that there is no environmental difference between ECF and TCF while others concluded that among ECF and TCF effluents before and after secondary treatment, TCF effluents are the least toxic.19, 20

A very large amount of research has been undertaken in laboratories internationally, comparing TCF bleaching using ozone with elemental chlorine free (ECF) bleaching using chlorine dioxide. These studies have shown that ECF bleaching produces much lower levels of organochlorine by-products than older bleaching methods that use elemental chlorine. When ecotoxicological studies are done on effluents from ECF and TCF bleaching following proper biological treatment (including microanalysis for the “dioxins”, PCDF and PCDD), both types of effluent show very low levels of toxicity that is removed by adequate dilution in the sea.15

Global Regulatory Position

These findings were independently reviewed by the United Nations Environment Program (UNEP), which concluded in 2003 that ECF and TCF bleaching methods are equivalent with respect to their potential formation of PCDD and PCDF (dioxins and furans). Based on this determination, neither process is considered to pose a greater dioxin-related risk than the other from a regulatory or environmental chemistry perspective. As a result, both ECF and TCF bleaching of kraft pulps are formally classified as Accepted Modern Technology in both the European Union and North America.15

Impact on Baby Health, Safety, and Skin

From a direct baby safety perspective, the bleaching method used to manufacture pulp does not create a meaningful difference in health risk, skin irritation, or toxicological exposure. More than 90% of human dioxin exposure comes from food—primarily meat, dairy products, fish, and shellfish—not from contact with paper or hygiene products. Modern exposure assessments show that any theoretical exposure from diapers is orders of magnitude below dietary exposure.

Modern medical and dermatological research on diaper dermatitis focuses on moisture, friction, urine and fecal enzymes, fragrances, adhesives, dyes, and surface treatments—not on pulp bleaching chemistry. Therefore, ECF versus TCF does not change diaper safety for a baby’s skin.

Why TCF Marketing Is Often Greenwashing

When diaper brands market Totally Chlorine Free (TCF) bleaching as if it provides superior baby safety, this constitutes a form of environmental greenwashing. The claim improperly converts a factory-level wastewater chemistry distinction into a consumer-facing health and safety narrative, despite the absence of any credible toxicological or exposure-based evidence supporting such a link. While TCF and ECF differ slightly in the types and quantities of chlorinated compounds measurable in mill effluent, these differences do not persist into the finished diaper at biologically meaningful levels, nor do they result in different infant exposure risks.

Toxicology does not evaluate hazard based on how a material was manufactured, but rather on actual exposure at the point of use. In the case of diapers, extensive environmental chemistry, industrial monitoring, and regulatory reviews consistently show that dioxins are either non-detectable or present at trace levels far below any threshold of health concern, regardless of whether the pulp was bleached using ECF or TCF methods. Moreover, over 90% of human dioxin exposure occurs through the food supply—not through skin contact with paper or hygiene products—making the theoretical contribution from diapers vanishingly small by comparison.

By framing TCF as a baby safety upgrade, marketing campaigns conflate environmental process preferences with product toxicological outcomes, creating an emotionally compelling but scientifically unsupported message for parents. This misrepresentation can also obscure the reality that nearly all global medical-grade and hygiene pulp—used in hospitals, NICUs, and surgical applications—is produced using ECF technology, under strict regulatory oversight with proven safety performance.

More importantly, the emphasis on bleaching method distracts from the factors that actually influence diaper comfort and skin health, including moisture control, rewet performance, breathability, friction, surface chemistry, urine and fecal enzyme exposure, and the presence of fragrances, dyes, adhesives, lotions, and surface treatments. These are the variables that dermatology and pediatric research consistently identify as drivers of diaper rash and irritation—not pulp bleaching chemistry.

In this way, TCF-centric safety marketing does not merely overstate an environmental preference; it redirects parental attention away from the product attributes that truly matter for infant comfort and skin protection, while simultaneously implying a health benefit that is not supported by exposure science, toxicology, or regulatory consensus.

Myth vs. Fact: ECF, TCF, and Baby Safety

MYTH: TCF diapers are safer for babies than ECF diapers. 

FACT: Both are toxicologically equivalent for baby safety.

MYTH: ECF diapers contain dangerous dioxins. 

FACT: Modern ECF produces non-detectable to trace levels far below health concern.

MYTH: TCF prevents diaper rash. 

FACT: Rash is driven by moisture, friction, and irritants—not bleaching chemistry.

MYTH: If it’s not TCF, it must be unsafe. 

FACT: 95% of the world’s hygiene pulp is ECF and regulator-approved.

Conclusion

ECF and TCF are both modern, regulated, and environmentally managed bleaching technologies. The difference between them is primarily environmental at the manufacturing level and does not translate into any measurable differences in diaper safety, skin exposure, or infant health. Marketing TCF as a baby safety upgrade misrepresents the actual science.

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