Market and product

Chlorine, a familiar element in the periodic table, is undergoing renewed scrutiny from the scientific community. As a pale greenish-yellow gas, chlorine forms the foundation of numerous industrial products, yet it also carries significant toxicity and environmental risks.

In a recent review published in ChemSusChem, a research team led by Professor Sebastian Riedel of the Free University of Berlin provided an in-depth analysis of the dual role of chlorine and hydrogen chloride (HCl)—one of its by-products. The authors emphasized chlorine’s major contribution to the production of pharmaceuticals, plastics, agrochemicals, and disinfectants, while also calling for a reassessment of its use due to toxicity and environmental impacts. Chlorine-free technologies, such as the hydrogen peroxide–propylene oxide (HPPO) process and polychloride-based ionic liquids, are emerging as safer alternatives that could even integrate with renewable energy systems.

Since ancient times, chlorine has been known by a name derived from the Greek word for “pale green.” In nature, it exists primarily as chloride salts in rocks and seawater and is essential for most living organisms. However, elemental chlorine is highly reactive, making it the “backbone” of the chemical industry—yet also a source of serious problems when released into the environment.

In public perception, chlorine is often associated with danger. It was used as a chemical weapon during World War I, alongside phosgene and mustard gas, causing thousands of deaths. Industrial accidents, such as the 1976 Seveso disaster in Italy—where toxic dioxin spread into the environment and injured 193 people, mostly children—remain haunting reminders.

Chlorine’s contradictions are also evident in agriculture. Chlorinated chemicals tend to be more persistent, providing effective crop protection without frequent application. A prominent example is DDT, developed by Swiss chemist Paul Hermann Müller, who received the Nobel Prize in Physiology or Medicine in 1948. DDT saved millions of lives by controlling malaria-carrying insects. In India, malaria cases dropped from 100 million in 1933 to just 0.15 million in 1966 thanks to DDT. In its first decade of use alone, it prevented 100 million infections and saved 5 million lives.

However, DDT also harmed the environment by accumulating in food chains, leading to declines in bird populations in North America and Europe. Since the 1970s, it has been banned for agricultural use in many countries, though still permitted for malaria control in some poorer nations. Scientists argue that eliminating DDT entirely requires the development of more environmentally friendly and affordable pesticides.

Another positive aspect of chlorine is its role in drinking water disinfection. For over a century, chlorine has eliminated disease-causing bacteria such as cholera, typhoid, and hepatitis A and E.

According to the World Health Organization (WHO), 1.7 billion people worldwide still lack access to safe drinking water, leading to 500,000 deaths annually. Chlorine significantly reduced epidemics in the United States and Europe from the late 19th century onward. Today, chlorination remains widespread in developed countries. Although concerns exist regarding chlorinated disinfection by-products (DBPs), WHO affirms that the public health benefits of chlorine outweigh the risks.

In the chemical industry, chlorine influences nearly every aspect of modern life. More than 50% of industrial chemicals and polymers depend on chlorine, including PVC plastics, polyurethane (PU), polycarbonate (PC), and silicones. Chlorine is essential for producing silicon used in semiconductor chips and solar panels, as well as titanium dioxide and phosphorus trichloride. Over 90% of pharmaceuticals and 86% of agrochemicals either contain chlorine or are synthesized using it.

In Europe, 7.3 million tons of chlorine were produced in 2023, mainly for PVC (32%) and PU/PC (30.7%). The chlor-alkali electrolysis process produces chlorine alongside hydrogen and sodium hydroxide (NaOH), requiring high electricity input—about 2.6 MWh per ton of chlorine. In Germany, the industry consumed 1.7% of national electricity in 2022, generating CO₂ emissions comparable to the steel sector. Nevertheless, chlorine remains inexpensive (around €200 per ton in 2019) due to abundant raw materials—salt and water—and its strong oxidizing power.

However, chlorine and NaOH production are tightly linked, meaning that reducing chlorine use requires alternatives to NaOH. Chlorine-based products often release HCl or chloride salts, which can pollute rivers if untreated. In 2022, elevated chloride concentrations combined with other factors triggered a toxic algal bloom in Germany’s Oder River, leading to mass fish deaths.

Chlorinated organic compounds such as DDT, hexachlorocyclohexane, and PCBs are often toxic and bioaccumulative, measured through the AOX (adsorbable organic halogens) index. Although nature produces more than 5,000 halogenated compounds, industrial sources dominate AOX levels. TOC (total organic carbon) is another indicator of organic pollution, and reducing chlorine use could lower both indices.

Safety remains the greatest challenge with chlorine and HCl. Chlorine is toxic and corrosive, stored under pressure (6.8 bar at room temperature), and can form heavy gas clouds if leaked. Transport poses high risks, as seen in a 2022 accident in Aqaba, Jordan, which killed 13 people and injured over 250. Chlorine is non-flammable but can explode when mixed with hydrogen, hydrocarbons, or ammonia. HCl presents similar hazards, stored at 42.6 bar and produced via chlorine–hydrogen reactions above 2,000°C. Pool-related chlorine accidents highlight the need for strict training and maintenance.

Given these disadvantages, the review calls for reducing chlorine use for sustainability and safety reasons. Complete replacement is difficult due to chlorine’s low cost and central role in pharmaceuticals, agrochemicals, and polymers. However, chlorine-free processes are being explored, drawing on reports from the German Environment Agency.

The review focuses on major chlorine-consuming chemicals in Germany in 2017:

Propylene oxide (PO, 1.1 Mt chlorine/27%)
Phosgene for PU (820 kt/20%)
Epichlorohydrin (ECH, 327 kt/8%)
Chlorinated methanes (CMDs, 222 kt/5%) for solvents and TFE
Phosgene for PC (122 kt/3%)

PVC (907 kt/22%) was excluded, as the final product contains chlorine and its production is already optimized.

For PO, primarily used in PU and polyester, Europe produced 2,860 kt in 2017. Germany’s chlorohydrin process consumes 1.35 tons of chlorine per ton of PO, generating saline waste. The MTBE co-production process avoids chlorine but depends on MTBE market demand. The HPPO process uses hydrogen peroxide to oxidize propylene, reducing salt waste by 70–80% and energy consumption by 35%, and is already implemented in Belgium and South Korea. However, it is more expensive and dependent on hydrogen peroxide production via anthracene.

Similarly, phosgene for PU and PC can be replaced by non-phosgene routes using dimethyl carbonate or urea, reducing toxic risks. ECH for epoxy resins can be produced from bio-based glycerin without chlorine. Chlorinated methanes for TFE (Teflon) could be replaced by direct methane fluorination, though costly. Polychloride ionic liquids may offer safer chlorine storage at room temperature and potential indirect energy storage applications.

Overall, chlorine is unlikely to be completely replaced in the near future. However, chlorine-free innovations and safer technologies are poised to reshape the chemical industry toward greater sustainability. Based on German data, the review suggests that these solutions could be applied globally, helping reduce environmental and health risks.