Chlorinated Organic Substances Can Be Converted into Dioxins in the Atmosphere

Are the Risks of Hazardous Chemicals Being Determined Appropriately?

In certain cases, apparently not. Reearcer from China indicate that chlorinated volatile organic compounds on mineral dust particles in the atmosphere can be converted into highly toxic polychlorinated dibenzo-p-dioxins and dibenzofurans by sunlight.

Hazardous chemicals are frequently assessed under national and international regulatory frameworks, which primarily focus on their toxic properties, environmental persistence, and accumulation in organisms. Yet, any transformations they undergo in the atmosphere are rarely or not at all taken into account, though secondary conversion products may show higher toxicity and persistence. These should be investigated to adequately evaluate risks posed by commercial chemicals to the environment and human health.

Chlorinated Volatile Organic Compounds (CVOC) Are Dioxin Precursors in Industrial Combustion

In this capacity, a team led by Xiaole Weng at Zhejiang University,Hangzhou, China, has examined chlorinated volatile organic compounds (CVOC). These important commercial chemicals are widely used in industry and agriculture, including in paints and varnishes, dry cleaning, and paint stripping. Waste incineration plants and landfills are also significant CVOC emitters. Growing industrialization will increase CVOC emissions still more, especially in developing countries.

CVOCs are known to be precursors to dioxin compounds in industrial combustion processes. Catalyzed by flue ash, for example, chlorobenzenes can be converted into polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Many compounds in this group are toxic and carcinogenic, as vividly demonstrated by the devastating chemical accident in Seveso in 1976. However, there is little data about the persistence and potential chemical conversions of CVOCs in the atmosphere.

Overlooked Source of Dioxins

Atmospheric particles contain substances like iron and aluminum minerals, which can be catalytically active. The team hypothesized that these catalyze the conversion of CVOCs into PCDD/Fs under solar radiation, acting as an important, overlooked source of dioxins.

To test this hypothesis, the researchers carried out laboratory experiments on a variety of mineral particles and identified possible reaction pathways based on computer models. A subsequent field trial with ambient air and fallen ash in an industrial park confirmed the occurrence of these photochemical conversions in the atmosphere.

The results prove that ubiquitous CVOCs, such as monochlorobenzene, dichloromethane, and perchloroethylene could be the overlooked precursors for PCDD/Fs. Iron oxides (α-Fe₂O₃) in particular, play a role in the production of chlorophenols and dioxin compounds. Tests on mice also demonstrated that after these photochemical reactions, the iron oxide dust caused severe damage to lung and brain tissue.

According to the researchers, this study underlines the need to reassess the toxicity of atmospheric precursor pollutants like commercial CVOCs, as well as their conversions.