Electronic cigarettes (also known as e-cigarettes, e-cigs, electronic nicotine delivery systems, electronic non-nicotine delivery systems, alternative nicotine delivery systems, personal vaporisers, e-hookahs, vape pens or vapes) are electronic devices that heat a liquid (or e-liquid) to produce an aerosol to inhale (known as vaping) [1]. The evidence on e-cigarettes is constantly evolving. E-cigarettes are relatively new products and most of the direct evidence on e-cigarettes is limited to short-term studies. Indirect evidence, such as studies that examine the long-term health effects of substances used in e-liquids, such as formaldehyde, can be useful to obtain a complete picture of the safety and potential long-term impacts of e-cigarette use.

The e-liquids are commonly made by combining flavouring chemical mixtures in a solvent mixture [2]. E-cigarette e-liquids can contain nicotine, but have been reported to also contain flavourings and harmful substances such as heavy metals, volatile organic compounds and cancer-causing chemicals [3] [4]. Recent reports have found over 200 unique ingredients (constituents) used in e-liquids [5] [6] [3]. These reports [5] [3] have also found that the known chemicals currently used in e-cigarettes are also used for other purposes, such as food processing, medicines and in the manufacture of other consumer goods. These chemicals have most likely undergone toxicological risk assessments. However, these risk assessments may not be comprehensive and risks to health via inhalation is not commonly addressed in food or medicine assessments [7] [8].

Given the established evidence on the safety and health impacts of nicotine and the wide range of non-nicotine substances in e-liquids and potentially inhaled via e-cigarettes, this report will consist of two components:
1. A summary of the toxicological risk assessments of each chemical currently known to be used in e-cigarettes. Objectives: (i) to examine the toxicological risk assessments and identify the health risks of chemicals known to be currently used in e-cigarettes and (ii) to identify if any of the known chemicals are permitted to be used (in Australia) for consumption via food and/or medicine and whether they have been assessed for inhalation toxicity.
2. A scoping review to ascertain the extent, range, and nature of the evidence available on toxicology associated with the inhalation of e-liquids. Objective: to examine the extent (that is, size), range (variety) and characteristics of the evidence published since 2019 on the inhalation toxicity of e-liquids used in electronic nicotine delivery systems and electronic non-nicotine delivery systems and highlight key research gaps.

Key findings
Toxicology assessments: According to toxicological assessment data, the majority of ingredients (chemicals) known to be used in e-cigarettes were associated with at least one or more health risk or suspected health risk (68.5%; n=253/369), including known or suspected acute toxicity (n=39), known or suspected carcinogen (n=82), known or suspected skin irritant (n=53), known or suspected skin sensitiser (n=114), and known to be harmful if swallowed (including fatal if swallowed; n=57). Of the chemicals known to be used in e-cigarettes, we identified 1 chemical permitted to be used in food in Australia by FSANZ and 4 chemicals classed as permissible ingredients for use in medicine by TGA that are considered harmful to inhale. Toxicological assessment data has an important role in identifying potential risks and identifying appropriate uses and handling for chemicals [13]. Whilst toxicological data was available for some of the chemicals, the comprehensiveness of this data, particularly for inhalation toxicity, is unclear as chemicals have not been assessed for this use (inhalation), but for other uses such as the manufacture of other consumer goods and ingestion as a component of food or medicine [14]. A large proportion of chemicals examined did not have toxicological assessment data available on inhalation toxicity (88.6%; n=327/369). It is important to note that the absence of data does not necessarily equal the absence of hazard.

Scoping review: After screening, 89 studies were included in the scoping review component of this report. Considerable variability in outcome of interest, study design and methodology was identified. The majority of included studies were experimental studies (65.2%; n=58/89 excluding randomised trials involving humans), however the type of experimental study (e.g. animal cell-based, biochemical) varied. Of the remaining study designs, just under a fifth were systematic reviews (19.1%; n=17/89). Cytotoxicity was the most common outcome of interest examined (n=24/89), followed by general toxicity (n=16/89), pulmonary toxicity (n=6/89) and cardiotoxicity (n=6/89). A number of the included studies examined e-cigarettes or e-liquids as a whole rather than specific constituents (12 of 89 studies examined individual constituents). The evidence on the differential health impacts of a specific flavour, solvent or humectant could not be determined. The evidence on the differential health impacts of nicotine-containing or nicotine-free e-cigarettes or e-liquids could also not be determined. It was common for studies not to specify whether the exposure (e.g. e-cigarette, e-aerosol, e-liquid) was a commercial device or e-liquid, whether it was a specific flavour, or whether or not it contained nicotine. Long-term data on inhalation toxicity of e-cigarettes (both nicotine-containing and non-nicotine-containing) remains limited. Time and resource constraints meant it was not feasible to conduct a systematic review of all identified e-liquid constituents.

Conclusions: In line with other published literature, this report found that where data was available, the majority of chemicals currently known to be used in e-cigarettes are associated with health risks, based on toxicological assessments. A large proportion of chemicals examined did not have toxicological assessment data available on inhalation toxicity and it cannot be concluded that absence of toxicological assessment data equates to absence of harm. Considerable variability in the outcomes of interest, exposure, study design and methodology was identified through the scoping review. Further research is required. Standardised methods for evaluating e-liquids are required, including specifications on device type, base liquid, concentration, coil/heating temperature and puff size. Such standardisation would lessen the many complexities of assessing toxicological health risks of inhaling non-nicotine e-cigarette constituents.