## 5 Conclusions  
From the current review of reactive and additive modifications of styrene polymers with P-containing compounds, it is possible to draw some general conclusions about their efficiency as fire retardants. Owing to the presence of chemically reactive aromatic rings, polystyrene chains are subjected to varying degrees of cross-linking reactions during post-polymerisation modifications. Whilst the \(co\)-polymerisation reaction might provide a more technologically attractive alternative, the choice of a suitable monomer often becomes a limiting factor, especially in the large-scale preparations. Therefore, the additive route is generally considered as more viable from a commercial point of view. However, this route is often associated with relatively high loadings to achieve an acceptable degree of fire retardance to styrene polymers. The use of the additive P-containing fire retardants remains very popular due to the simplicity of this approach. Many studies carried out in recent years were focussed on varying the ratios of the components in the additive FRs, on the incorporation of the nanomaterials with varied chemical constitutions and of different origins, and then on monitoring the thermal parameters (decomposition temperature, heat release rates, char yields, etc.). Even though these studies have demonstrated an increase in the overall fire-retardant properties, as identified through some prescriptive laboratory-scale techniques/tests, they still remain predominantly in the realm of an academic pursuit. Some compositions such as graphene-based ones are rather complex and expensive; their poor compatibility and the unknown environmental and health impacts prevent their wider industrial use. The problems associated with high loadings of additive FRs can be reasonably solved by adopting the reactive modifications or through the use of synergists, which are frequently combined with nanoclays, carbon nanotubes, or graphene/graphite compositions. However, only a limited number of publications reported the successful suppression of smoke or toxic gases released during burning of the modified styrene products. From a practical point of view, there are some other challenges that need due consideration while formulating any FR strategy, including sustainability, cost-effectiveness, and the ease of processing, and hence the commercial viability of the proposed formulations; the environmental implications of the nanofillers, additives, or monomers; the overall efficacy of the modifying compounds in the case of a fire; etc. None of the reviewed publications had systematically studied the toxicity of the gaseous products in the fire effluents of the modified styrene compositions. As the use of P-containing FRs becomes more widespread, their impact on public health and the toxicity of their decomposed fragments should be the key tasks for the researchers in the near future. There are also some serious concerns related to a loss of FRs during the life cycle of styrene products, potentially causing harm to the environment and negatively affecting human health. The future trends will likely involve the greater use of bio-inspired fire-retardant solutions or P-containing waste materials [126; 127; 128; 129]. However, only a small number of bio-based P-containing formulations were proposed recently, and they were required at very high loadings [127; 128]. The effectiveness of phosphorus-based compounds depends on various parameters such as the P-content present in the polymeric material, the chemical environment of the P atom, its position in the polymeric chain, the oxidation state, and the distribution of the modification within a host polymeric matrix. The chemical nature of the groups covalently attached to the phosphorus atom also greatly influences the fire-retarding properties of P-bearing FRs. Generally, phenyl or aromatic groups bonded to the P atom could assist in maintaining the higher thermal stability of the fire-retarded styrene polymers.