**1. Introduction**

Materials with high carbon and hydrogen content are easily combustible, so most materials used nowadays, including plastics, are flammable. Over the last century, the furniture, electronics, upholstery, and textile industries have increasingly employed synthetic materials, which are also used in the transport sector (cars, airplanes, and trains) and at home. Using safety devices against fire, like flame retardants, is important to prevent these materials from burning and harming the society and the environment [1, 2].

But what are flame retardants? Flame retardants (FRs) are chemical compounds employed as safety devices to prevent fires from starting/spreading or to delay ignition, thereby reducing combustible material flammability, increasing escape time, and providing safety to humans and properties [3, 4]. The term "flame retardants" refers to the chemical compound action and not to the compound itself [5]. Various chemical compounds with different physicochemical properties and molecular structures can act as FRs. They can be added to (additive FRs) or incorporated into (reactive FRs) combustible materials, such as wood, plastics, kitchen utensils, appliances, computers, electrical cables, construction materials, textiles, and upholstery [6].

The global FR market is expected to reach about US\$53 billion by 2024. In 2019, the world FR consumption amounted to over 2.4 million tons, corresponding to 4.9% growth in market size [1, 7, 8]. China is the largest FR consumer—it accounts for 26% of the global consumption, followed by Western Europe (23%), North America (22%), Asia (18%), and Japan (6%). Together, Central/Eastern Europe, Central/South America, and Middle East/Africa add up to 5% of the world's consumption. Over 175 chemicals are listed as FRs. They are classified on the basis of their chemical composition, but a single compound, aluminum trihydroxide (Al(OH)3), tops the list as the most consumed FR in the world, corresponding to 38% of all the FRs consumed worldwide. Halogenated flame retardants (HFRs) come next (21%, being 17% brominated FRs and 4% chlorinated FRs), followed by organophosphorus (18%). Other classes like metal-based FRs amount to 14% of the global consumption, followed by FRs based on antimony oxides (9%) [7, 8]. **Figure 1** summarizes the consumption of flame retardants.

Despite the recent increase in FR use, the first reports on their application date back to 450 BC., when Egyptians employed aluminum to reduce wood flammability. Reports dating back to 200 BC. describe that the Roman civilization used aluminum with vinegar to decrease wood flammability [9]. In modern times, specifically in 1929, polychlorinated biphenyls (PCBs), the first class of FRs, were introduced in the United States of America to meet the need of the electrical industry for an insulator that could act as FR. Later, Europe and Japan also started to produce PCBs. After 37 years, PCB presence in the environment was reported for the first time: a Swedish biologist detected PCBs in fish. Two years later (1968) in Japan, about 1000 Japanese were intoxicated with rising oil contaminated with PCBs. PCBs were widely applied until the 1970s. Then, they were banned in Japan in 1972, and North America stopped producing them in 1976 [1, 10]. However, PCB presence in the environment is still relevant because they are Persistent Organic Pollutants (POPs) with the ability to bioaccumulate and biomagnify, consequently presenting high toxic potential [9, 11].

After PCBs were banned, brominated flame retardants (BFRs) emerged as an economically viable alternative to replace them. Although BFRs and PCBs differ because they belong to distinct chemical classes, the BRF mechanism of action resembles the PCB mechanism of action. In the gas phase, brominated and chlorinated FRs inhibit the combustion process of root chain reaction. HFRs neutralize the high-

#### **Figure 1.**

*Global consumption of flame retardants and their consumption by classes. (Designed using GraphPad prism 8.0.2. Adapted form FlameRetardant-online [7]).*

energy OH˙ and H˙ radicals originating from a chain reaction in fire [12, 13]. However, concerns about HFR toxicity have been raised because they may leach into the environment, with high HFR concentrations being recorded in fish and marine mammals. Concerns about BFR toxic and ecotoxic effects, mainly their carcinogenic and endocrine-disrupting actions in humans, have pressed authorities to legislate about or even ban some BFRs. For example, commercial mixtures of polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane (HBCD) have been banned or phased out in North America and the European Union (E.U.) [14]. With the new legislation regarding BFRs, the global market has sought economically viable and environmentally friendly alternatives that act similarly to banned FRs. In this context, phosphorus flame retardants (PFRs) have emerged as suitable alternatives for BFRs although they have already been employed for over 150 years [4, 6].

Concerns about FRs being present in the environment grow every day. FRs may easily spread to environmental compartments (air, water, soil, sediments, and even house dust) through dissolution, volatilization, and attrition [4]. Improperly disposed electronic waste and furniture contribute to FR presence in the environment. Weak chemical interaction between manufactured products and FRs applied to them aggravates FR dispersion in the environment, not to mention that numerous compounds employed as FRs have serious effects on human health and the environment. Therefore, ensuring conscious use of these chemicals is crucial [14–16].
