New contaminants in indoor environments: occurrence, transformation, and health risks

People spend approximately 90% of their time in various indoor environments, such as homes, offices, schools, and shopping malls^[1]. Therefore, understanding and predicting indoor environmental quality is crucial for human health. In indoor environments, the composition and sources of harmful pollutants are complex and diverse, and long-term or repeated exposure to these pollutants can lead to serious adverse health effects^[2]. For instance, previous studies have shown that indoor air pollution can contribute to cardiovascular diseases and respiratory disorders, including cancers of the lung, bronchus, and trachea^[3]. According to the World Health Organization, millions of people die prematurely each year due to indoor air pollution^[4]. In China, indoor air pollution contributed to 14.1% of the total disease burden in 2017, ranking third among all identified health risk factors. The associated economic losses were estimated at CNY¥2.88 trillion, representing approximately 3.45% of the national GDP^[5]. Therefore, improving indoor environmental quality is crucial for protecting human health.

Indoor environmental quality has attracted increasing public attention in recent years. For example, the Chinese government has actively advanced initiatives to improve indoor air quality^[6]. In 2022, China officially implemented a new national indoor air quality standard, which introduced threshold values for trichloroethylene, tetrachloroethylene, and fine particulate matter, while lowering the permissible limits for nitrogen dioxide, carbon dioxide, formaldehyde, benzene, and inhalable particulate matter^[7]. This revision represents a significant step toward the establishment of high-quality indoor environments. Although the new standard introduced three additional compounds, limit values have been established for only 18 substances in total. With growing awareness of the health risks associated with chemical products, the presence of new contaminants has been reported, including persistent organic pollutants, endocrine-disrupting chemicals (EDCs), antibiotics, and microplastics^[8]. Compared with traditional pollutants, these new contaminants are diverse in type and characterized by latent risks, posing potential threats to human health^[9]. Furthermore, due to their environmental persistence and mobility, they tend to accumulate and persist in indoor environments, resulting in long-term human exposure^[10,11]. Therefore, exposure to new contaminants in indoor settings warrants considerable attention.

Indoor environments contain a wide range of chemical products, many of which can serve as sources of new contaminants^[12,13]. For example, common personal care products such as shampoos, body washes, sunscreens, and nail polishes contain EDCs like bisphenol A and phthalates^[14]. Plastic products, including packaging materials, garbage bags, and beverage bottles, are rich in microplastics such as polyethylene, polypropylene, and polystyrene^[15]. In addition, materials such as carpets, flooring, and wall paints are significant sources of perfluorinated compounds^[16]. These new contaminants can be released into the indoor environment through the regular use, volatilization, abrasion, or aging of chemical products, leading to their widespread presence in indoor air, dust, and surface materials^[17]. They can subsequently enter the human body via ingestion, dermal contact, or inhalation, posing potential risks to human health^[18].

Current research on new contaminants has primarily focused on outdoor environments, while studies on indoor environments are still relatively limited^[19]. Notably, to reduce energy consumption and enhance comfort, modern buildings are designed with higher airtightness and lower indoor–outdoor air exchange. As a result, pollutants from indoor sources can easily accumulate, resulting in prolonged human exposure^[20]. More severe air pollution indoors compared to outdoors has also been frequently reported^[21]. Given the substantial contribution of indoor exposure to total human exposure and the tendency of new contaminants to accumulate indoors, research should focus on the pollution characteristics and health risks of these contaminants in indoor environments to improve indoor environmental quality and protect human health.

Similar to traditional pollutants, the emission sources of new contaminants vary considerably among different types of indoor environments, resulting in substantial differences in their composition and concentrations (Fig. 1)^[22]. Therefore, identifying the major emission sources in different indoor environments is a fundamental prerequisite for effective control of new contaminants. Common building and decorative materials in indoor environments, such as furniture, paints, and wallpapers, can release phthalates, polybrominated diphenyl ethers, and organophosphate esters (OPEs), resulting in generally high levels of these compounds, particularly in newly renovated buildings^[23−25]. Occupational environments often contain high concentrations of specific pollutants. For example, cooking processes and tobacco smoke are the primary sources of nitrosamines^[26]. Therefore, higher concentrations of indoor nitrosamine pollution are likely to occur in places with intensive cooking activities, such as restaurants and kitchens, as well as in smoking environments like mahjong parlors^[27]. Pharmaceuticals and personal care products are important new contaminants in residential environments, as they are widely used in cosmetics, therapeutic drugs, and household pest control products^[28]. In daycare centers, polyvinyl chloride floor mats, foam cushions, and toys contain substantial amounts of antioxidants, plasticizers, and flame retardants, which can be gradually released into the indoor environment as these products age^[29]. In offices, computer rooms, and other fire-sensitive areas, building materials typically contain high levels of flame retardants and plasticizers^[30]. The high density and frequent replacement of electronic devices further contribute to the emission of OPEs and alternative halogenated flame retardants, making these environments significant sources of such contaminants^[31]. Variations in chemical product usage across occupational settings result in differences in the types and concentrations of dominant new contaminants indoors. Therefore, identifying the characteristic new contaminants specific to each type of workplace can support the development of targeted prevention and control strategies, thereby enhancing the efficiency of pollution management in various indoor environments. Furthermore, the diverse sources of new contaminants and the wide variation in indoor layouts make it challenging to accurately model indoor new contaminant pollution. Accurate simulation of indoor new contaminant pollution requires comprehensive identification of key pollutants across different functional indoor environments.

Figure 1. 

Schematic diagram of new contaminants in indoor environments.

In outdoor environments, high photochemical activity, abundant oxidants, and a complex composition of substances contribute to the transformation of new contaminants into more persistent and toxic secondary products through multiple pathways^[32]. Similar characteristics have also been observed in indoor environments. Ultraviolet radiation from indoor lighting and ozone produced during ventilation or appliance use, can induce photooxidation^[33]. Consequently, chemicals released from consumer products, including organophosphorus flame retardants emitted from furniture, electrical appliances, and building materials, may be transformed into oxidized derivatives with greater neurotoxic potency^[34]. In addition to photooxidation, gas-phase oxidation reactions within the indoor atmosphere are also a common transformation pathway^[35,36]. For example, linalool, a new contaminant released from chemicals such as cleaning agents, disinfectants, and cosmetics, can undergo gas-phase oxidation reactions with ozone or hydroxyl radicals to produce a variety of oxidative products, including carbonyl compounds and secondary organic aerosols. These oxidative derivatives often exhibit greater oxidative reactivity and toxicity potential than the parent compound, thereby increasing the risk of respiratory and systemic adverse effects^[37].

A key characteristic of indoor environments is their high surface-to-volume ratio, which not only enhances heterogeneous reactions but also increases the chemical complexity of indoor pollutants^[20]. For instance, di(2-ethylhexyl) phthalate, a plasticizer extensively released from polyvinyl chloride flooring, vinyl materials, and consumer plastics, can generate gas-phase hydroxyl radicals on indoor surfaces and within dust films, thereby initiating heterogeneous oxidation processes that yield numerous oxygen-containing derivatives^[38]. These oxidation products have higher chemical stability and a stronger endocrine-disrupting potential than the parent compound, resulting in elevated health risks^[39]. The transformation of new contaminants is both a key source of indoor pollution and a major factor in heightened human health risks. Therefore, research on indoor contaminants should consider the distinctive features of indoor environments, such as the high surface-to-volume ratio, to differentiate indoor from outdoor processes better.

New contaminants in indoor environments can enter the human body through inhalation, ingestion, or dermal absorption, thereby posing potential health risks^[40]. For example, EDCs contained in personal care products such as sunscreens and skin-whitening agents can penetrate the skin barrier and enter systemic circulation^[41]. New contaminants have been widely detected in various human tissues and biological matrices, such as bone marrow, blood, urine, hair, breast milk, nails, and adipose tissue^[11]. For instance, microplastics have been identified in human bone marrow samples, while urinary analyses have frequently revealed the presence of bisphenol A, OPEs, phthalates, and organophosphate pesticides^[42−44]. The adverse health effects of new contaminants have been extensively reported, including but not limited to: endocrine-disrupting toxicity, developmental toxicity, neurotoxicity, reproductive toxicity, and potential carcinogenicity^[45,46]. For example, Whyatt et al. reported that prenatal exposure to monobutyl phthalate and monobenzyl phthalate may negatively affect children's cognitive and behavioral development^[47]. Importantly, sensitive populations, such as the young (under 18 years old), and the elderly (over 80 years old), typically spend over 90% of their time indoors^[29]. Consequently, exposure to indoor new contaminants may pose significant health risks for these vulnerable groups. For example, studies have shown that exposure to new contaminants in indoor environments can adversely affect the health of subsequent generations^[42,43]. During pregnancy, these contaminants can cross the placental barrier and directly disrupt fetal development and may also indirectly impact fetal growth and increase the risk of adult diseases by modifying maternal intrauterine factors^[48]. For instance, Kim et al. found that maternal urinary concentrations of diethyl phthalate were positively correlated with adverse neurodevelopmental performances in children aged 1–2 years^[49].

Given the importance of the indoor environment for human health, especially for sensitive populations, future research should emphasize new contaminants in diverse indoor environments, such as residential, educational, medical, commercial, recreational, and sports facilities, which will support the establishment of a comprehensive database of indoor new contaminants. In addition, future studies should employ high spatiotemporal-resolution sampling to disclose shared pollutant characteristics and summarize pollution patterns from extensive datasets. A comprehensive understanding of the mechanisms and contributions of secondary formation processes is essential for developing effective pollution control strategies. Finally, toxicological and cellular studies should be conducted to evaluate the health risks of new contaminants, providing a scientific foundation for their regulation and the development of indoor environmental standards.