I. Introduction

Electronic cigarettes (e-cigarettes), or vaping devices, function by heating a liquid (e-liquid) to produce an aerosol that is inhaled by the user. While often marketed as a less harmful alternative to combustible cigarettes, the use of e-cigarettes, particularly indoors, raises public health concerns regarding bystander exposure to the exhaled aerosol, commonly referred to as secondhand vape aerosol (SHA). Unlike traditional cigarettes, e-cigarettes do not produce side-stream emissions between puffs; SHA consists primarily of the aerosol exhaled by the user. However, this exhaled aerosol contains residual nicotine, components of the e-liquid base (propylene glycol and vegetable glycerin), flavorings, and potentially harmful byproducts generated during the heating process.

Understanding the potential exposure levels for bystanders is crucial, especially in enclosed indoor environments where pollutants can accumulate. This report aims to estimate the amount of SHA inhaled by a non-vaping bystander under a specific, controlled scenario: presence for one hour in a closed room with a volume of 32 cubic meters (e.g., 4m x 4m x 2m) where an individual is vaping constantly. This estimation relies on synthesizing data from scientific literature regarding the composition and concentration dynamics of SHA in indoor settings and applying established exposure assessment principles. The report will review the chemical constituents of SHA, analyze factors influencing its concentration in indoor air, synthesize relevant concentration data from experimental studies, outline dose estimation methodologies, and finally, provide a quantitative estimate of the inhaled dose for key SHA components under the specified conditions, explicitly stating the underlying assumptions and limitations.

II. Chemical Composition of Secondhand Vape Aerosol (SHA)

The aerosol inhaled and subsequently exhaled by an e-cigarette user is a complex mixture derived from the e-liquid constituents and thermal degradation products formed during heating.¹ While SHA generally contains fewer toxicants than secondhand smoke (SHS) from combustible cigarettes, it is not merely harmless water vapor.

Key components identified in SHA include:

• Nicotine: Most e-liquids contain nicotine, an addictive stimulant.³ While vapers retain a significant portion of inhaled nicotine (over 99% reported in some lab studies ⁵), measurable amounts are still exhaled and contribute to SHA.⁶ Nicotine itself, while addictive and harmful to adolescent brain development and during pregnancy, is not considered the primary cause of smoking-related cancers.⁸ However, its presence in SHA indicates exposure to the aerosol mixture.
• Propylene Glycol (PG) and Vegetable Glycerin (VG): These are the primary base solvents in most e-liquids, considered safe for ingestion but less understood upon inhalation.¹ They form the bulk of the visible aerosol droplets.¹² Heating PG and VG can lead to the formation of carbonyl compounds.¹ PG exposure can cause eye, throat, and airway irritation.¹³ Substantial amounts of PG and VG have been measured in SHA.
• Flavoring Chemicals: Thousands of flavorings are used in e-liquids, many deemed safe for ingestion but not necessarily for inhalation.³ Some flavorings, like diacetyl (linked to bronchiolitis obliterans or “popcorn lung”) and cinnamaldehyde, have raised toxicological concerns when inhaled.¹¹ Flavorings contribute significantly to the chemical diversity of the aerosol.¹²
• Carbonyl Compounds: Thermal decomposition of PG, VG, and flavorings can generate aldehydes such as formaldehyde, acetaldehyde, and acrolein. Formaldehyde and acetaldehyde are classified as carcinogens.¹¹ Levels can vary significantly depending on the device power, voltage, and temperature, with higher temperatures potentially producing higher levels.¹ Some studies suggest formaldehyde levels can exceed those in cigarette smoke under certain conditions, though others find negligible release at normal settings.¹
• Volatile Organic Compounds (VOCs): Compounds like benzene (found in car exhaust) and toluene have been detected in SHA, though often at low levels.
• Metals: Trace amounts of metals, including heavy metals like nickel, tin, lead, and chromium, have been detected in SHA. These likely originate from the heating coil and other device components.¹ Concentrations of some metals in SHA have been reported to be higher than or equal to those in SHS.¹³
• Particulate Matter: SHA consists of an aerosol of fine and ultrafine liquid droplets. These particles fall predominantly within the PM2.5 size range (diameter < 2.5 micrometers) and even smaller (ultrafine, < 0.1 micrometers or 100 nm). Such small particles can penetrate deep into the lungs.

The exact composition and concentration of these constituents in SHA vary significantly depending on factors like the e-liquid formulation (nicotine strength, PG/VG ratio, flavors), the type of vaping device and its operational settings (power, temperature), and the user’s vaping behavior (puff duration, inhalation depth). Recent untargeted analyses suggest the chemical complexity is greater than previously thought, with potentially thousands of compounds present, many unidentified.¹ However, compared to the estimated 5,000+ chemicals in tobacco smoke, e-cigarette aerosol is generally considered less complex, with PG, VG, water, and nicotine comprising the vast majority (89-99%) of the aerosol mass.¹

III. Characteristics of Secondhand Vape Aerosol Indoors

When SHA is exhaled into an indoor environment, its behavior is governed by physical and chemical processes that differ somewhat from those of SHS.

• Particle Size and Number: SHA consists primarily of ultrafine liquid droplets, typically with diameters ranging from nanometers to a few hundred nanometers (e.g., 20-300 nm reported in one study ¹⁵, modes around 15 nm and 85 nm in another ¹⁶). Particle number concentrations (PNC) can be very high immediately after exhalation, potentially reaching levels of 10⁴ to 10⁵ particles/cm³ or higher near the source.¹⁶ Some sources state SHA particle concentration is higher than SHS.¹³
• Evaporation and Decay: Unlike the solid and tar-based particles in SHS, the liquid droplets in SHA (primarily PG and VG) evaporate relatively quickly, especially in typical indoor conditions.¹⁶ Studies in controlled chambers have observed particle concentrations returning to near-background levels within seconds to minutes after a puff or cessation of vaping, much faster than the decay of SHS particles which can linger for 30-45 minutes or longer.⁷ One study reported a half-life on the order of seconds. This rapid evaporation affects particle removal mechanisms, making evaporation dominant over ventilation or deposition for the liquid droplet phase.¹⁹ However, less volatile components like nicotine and metals, as well as gaseous components, persist longer and can accumulate.
• Spatial Distribution: Concentrations of SHA components are highest near the vaper and decrease with distance.¹⁶ One study measured mean PM2.5 concentrations during puffing of 188 µg/m³ at 0.8 meters, dropping to 19 µg/m³ at 1.5 meters.¹⁶ In multi-room settings, SHA particles and chemicals can migrate from the vaping area to adjacent non-vaping spaces, although concentrations are reduced.² Factors like ventilation and whether connecting doors are open or closed significantly influence this transport.¹⁷
• Thirdhand Exposure: Similar to SHS, components of SHA, particularly nicotine and potentially other semi-volatile compounds, can deposit onto indoor surfaces (walls, furniture, clothing) creating reservoirs for potential thirdhand exposure via dermal contact, dust ingestion, or re-emission into the air. Studies have detected nicotine and tobacco-specific nitrosamines (TSNAs, which can form from nicotine reacting with ambient nitrous acid) on surfaces in homes and vape shops where vaping occurs.² Nicotine accumulation rates on surfaces have been measured during short-term vaping sessions.⁷ The persistence and potential health risks of thirdhand vape exposure are still under investigation but represent another pathway for bystander exposure.²¹

IV. Synthesizing Concentration Data in Controlled Indoor Environments

To estimate bystander dose, it is essential to establish the likely range of airborne concentrations of key SHA components (PM2.5, nicotine, PG/VG) under conditions relevant to the query (closed room, ~32 m³, 1 hour constant vaping). Several studies have measured SHA concentrations in controlled indoor settings like rooms or chambers, providing valuable data points. However, direct comparisons are complicated by variations in room volume, ventilation rates, measurement duration, vaping protocols (device type, liquid, puffing regimen), and analytical methods.

The table below summarizes key quantitative findings from studies conducted in controlled rooms or chambers, focusing on PM2.5 and nicotine concentrations. Studies were selected for relevance to the query’s parameters (indoor room/chamber, specified volume/ventilation where available, measured SHA components).

Table 1: Summary of Measured SHA Concentrations in Controlled Indoor Environments

Study (Snippet ID)	Room Vol (m³)	Ventilation (ACH)	Duration / Protocol	Device/User(s)	Sampling Loc.	PM2.5 (µg/m³) Mean/Median	Nicotine (µg/m³) Mean/Median	PG/VG (µg/m³) Mean/Median	Notes
Goniewicz 2014 6	39	1.37 (Low)	1 hr machine vaping (various brands)	Machine (3 brands)	Chamber	33.1 ± 26.9 [6.6–85.0]	2.51 ± 1.68 [0.82–6.23]	NP	1-hr avg. CO, VOCs not sig. elevated. Nicotine ~10x lower than SHS (31.6 µg/m³).
Zhao 2017 16	NP	NP	10 min human vaping (ad libitum)	13 experienced users	0.8 m	188 ± 433 [Peak ~3000]	NP	NP	Mean during puffing. PNC peak ~10⁵ p/cm³. Rapid decay (<20s). Bkgd PM2.5=8.
Zhao 2017 16	NP	NP	10 min human vaping (ad libitum)	13 experienced users	1.5 m	19 ± 14	NP	NP	Mean during puffing. PNC mean 9.9×10³ p/cm³. Demonstrates distance effect.
Melstrom 2017 7	52.6	~5	2 hr human vaping (ad libitum)	Disposable EC users	Room	Median 35 [0.002–19,961]	Median 0.70 ng/L (≈µg/m³)	NP	Higher ventilation than query. PM2.5 uses mg/m³ units in source (0.035 mg/m³). Nicotine accumulation on surfaces measured.
Melstrom 2017 7	52.6	~5	2 hr human vaping (ad libitum)	Tank EC users	Room	Median 515 [0.007–19,972]	Median 1.83 ng/L (≈µg/m³)	NP	Higher ventilation. PM2.5 uses mg/m³ units (0.515 mg/m³). Shows large device impact. Nicotine accumulation measured.
Ruprecht 2018 19	35.8	0, 1, 2	Human vaping (protocol NP)	Smokers using EC	Bystander Loc	PNC measured, returned to bkgd <15s	NP	NP	Focus on particle decay time (fast evaporation) vs ventilation. PM mass not reported. Relevant volume & 0 ACH condition tested.
Tzortzi 2020	Room (NP)	NP	30 min human vaping (ad libitum)	1 user	Room	Median 21 (vs ~10 bkgd)	Mostly <LOQ	NP	Short duration. Also tested in car (median 16). Bystander irritation symptoms reported.
Zhang 2020 17	89	Low (NP)	12 min human vaping (controlled) – Door Open	NP	Vaping Room	64 (Mean)	NP	NP	Larger room. Focus on inter-room transport. Baseline condition.
Zhang 2020 17	89	Low (NP)	12 min human vaping (controlled) – Door Closed	NP	Vaping Room	Peak 69	NP	NP	Closing door increased PNC by 26% in vaping room.
Avino 2021 24	51.9	3.3 (Hospitality)	4 hr human vaping (prescribed & ad libitum)	Group I (JUUL)	Room	PNC measured (not PM2.5)	0.75±0.20 (prescr), 1.91±0.42 (ad lib)	PG: 5.9/9.5, VG: 17.8/32.3	4-hr cumulative avg. Higher ventilation than query. Provides PG/VG data.
Avino 2021 24	51.9	3.3 (Hospitality)	4 hr human vaping (prescribed & ad libitum)	Group II (VUSE)	Room	PNC measured (not PM2.5)	2.77±2.75 (prescr), 1.87±2.56 (ad lib)	PG: 0.25/BBV, VG: 2.8/1.9	4-hr cumulative avg. Higher ventilation.
Ballbe 2022 25	Home (Varies)	Varies	7 days continuous monitoring	Home user	Home	Similar to controls	Geo Mean 0.01 [0.01–0.02]	NP	Long-term avg in real homes, likely lower intensity/higher ventilation than query scenario. Nicotine quantifiable in 72% of homes.

NP = Not Provided/Not Measured; <LOQ = Below Limit of Quantification; BBV = Below Baseline Value; PNC = Particle Number Concentration; bkgd = background; prescr = prescribed use; ad lib = ad libitum use.

Analysis of Concentration Data:

The synthesized data reveal several important points pertinent to estimating bystander exposure:

1. Significant PM2.5 Elevation: E-cigarette use consistently increases indoor PM2.5 concentrations above background levels. Measured concentrations vary widely, influenced heavily by the type of device used. Studies using tank systems or measuring close to the source report higher levels (e.g., median 515 µg/m³ ⁷, mean 188 µg/m³ at 0.8m ¹⁶) compared to those using disposable devices or measuring further away or averaged over longer periods (e.g., median 21 µg/m³, 35 µg/m³ ⁷, 19 µg/m³ at 1.5m ¹⁶). Even the lower end of these measurements often exceeds 24-hour ambient air quality guidelines recommended by organizations like the WHO (e.g., 15 µg/m³). This wide range highlights that assumptions about the vaping device and intensity are critical for estimating PM2.5 exposure. A single vaper using a high-power device could generate substantial particulate pollution in a small, closed room.
2. Measurable but Lower Nicotine Levels: Airborne nicotine concentrations are detectable in environments where vaping occurs, but levels are consistently much lower than those found in comparable SHS environments.⁵ Typical 1-hour average concentrations in controlled room studies under low-to-moderate ventilation range from approximately 0.7 to 6 µg/m³.⁶ While long-term averages in homes are lower (around 0.01 µg/m³ ²⁵), short-term exposure in a confined space with constant vaping will result in higher concentrations within this range. The 10-fold lower nicotine concentration compared to SHS observed in one chamber study ⁶ suggests that while SHA is not nicotine-free, the nicotine dose received by a bystander is likely substantially less than from an equivalent duration of SHS exposure yielding similar PM2.5 levels.
3. Presence of PG/VG: While less frequently measured, PG and VG are major components of the aerosol mass.¹² One study reported 4-hour average concentrations in a 52 m³ room with ~3 ACH ranging from below baseline to ~10 µg/m³ for PG and ~2 to 32 µg/m³ for VG, depending on the device and usage pattern.²⁴ Concentrations would likely be higher in a smaller, unventilated room over 1 hour.
4. Influence of Room Conditions: Room volume and ventilation significantly impact concentrations. Lower ventilation rates (fewer air changes per hour, ACH) lead to higher accumulation of pollutants.²¹ Closing doors between spaces increases concentrations in the source room.¹⁷ The query specifies a relatively small volume (32 m³) and a “closed” state, implying low ventilation (likely < 1 ACH due to natural infiltration only ²⁹). These conditions favor higher concentrations compared to larger rooms or those with active ventilation (like the 5 ACH in ⁷ or 3.3 ACH in ²⁴).

V. Estimating Bystander Inhaled Dose

To estimate the amount of SHA inhaled by a bystander in the specified scenario, several methodologies can be considered.

A. Review of Dose Estimation Methodologies

1. Concentration-Based Approach: This is the most common and practical method for estimating potential inhalation exposure based on environmental measurements. It calculates the inhaled dose by multiplying the average airborne concentration (C) of a substance by the bystander’s breathing rate (BR) and the duration of exposure (T).28 Optionally, a deposition fraction (DF) can be included to estimate the amount deposited in the respiratory tract, rather than just inhaled. The basic formula is:
   Dose = C × BR × T × DF
   This method relies heavily on accurate measurement or estimation of the average concentration (C) the bystander breathes over the exposure period.
2. Modeling Approaches:

• Mass Balance Models: These models treat the indoor space as a system where the change in pollutant mass is determined by the source emission rate, removal by ventilation, removal by deposition onto surfaces, and potentially chemical decay.²⁸ By inputting parameters like room volume (V), air exchange rate (ACH), deposition velocity (vd), and source emission rate (E), these models can predict the concentration (C) over time.²⁸ They can simulate the build-up of pollutants from a constant source in a closed room, providing a more dynamic picture than a simple average concentration. Simplified formulas derived from mass balance principles can be used for estimation.³¹
• Computational Fluid Dynamics (CFD): CFD models provide a more sophisticated simulation of airflow patterns and pollutant transport within a specific room geometry.³² They can capture spatial variations in concentration, potentially predicting higher exposures for bystanders closer to the vaper. CFD can be coupled with physiologically based pharmacokinetic (PBPK) models to estimate internal tissue doses from inhaled or dermally absorbed substances.³² While powerful, these models require detailed inputs and computational resources, making them less suitable for a generalized estimate.

3. Biomarker Approach: This involves measuring biomarkers of exposure (e.g., cotinine in saliva or urine for nicotine exposure, metabolites of PG/VG) in individuals exposed to SHA. Biomarkers provide definitive proof of absorption but relating biomarker levels back to a specific inhaled amount of aerosol or its components is complex and requires pharmacokinetic modeling.³² This approach is more useful for confirming exposure occurrence and comparing relative exposures (e.g., SHA vs. SHS ⁵) than for directly estimating the inhaled dose of the aerosol itself in mass or volume units.

B. Application to the Specific Scenario (32 m³ closed room, 1 hr constant vaping)

For this report, the concentration-based approach (Dose = C × BR × T) is the most feasible method, utilizing the synthesized concentration data (C) from Section IV.

1. Method Selection: We will estimate the inhaled dose using Dose = C × BR × T. This provides a direct estimate of the potential amount inhaled based on air concentrations.
2. Concentration (C): Selecting appropriate average concentration values for PM2.5 and Nicotine over the 1-hour period is the most critical step. Based on the data in Table 1 and considering the scenario (32 m³, closed room ≈ low ventilation < 1 ACH, 1 hr constant vaping), we must make informed choices:

• Relevance of Studies: Studies ⁶ (39 m³, 1.37 ACH, 1 hr) and (room, 30 min) provide relevant benchmarks for low ventilation and shorter durations. ⁷ provides data differentiating devices but at higher ventilation (5 ACH) and longer duration (2 hr). ¹⁶ shows high near-source peaks but lower average levels further away. ¹⁹ confirms rapid decay in a similar volume at 0 ACH but didn’t report mass concentrations suitable for dose calculation.
• Impact of “Constant Vaping”: This implies a moderate to high emission rate. Tank systems generally produce higher PM2.5 than disposables.⁷
• Selected Range: To account for uncertainty, particularly regarding the intensity implied by “constant vaping” and device type, a range will be used.

• C_PM2.5: A plausible range for a 1-hour average in a 32 m³ closed room with constant (moderate to high intensity) vaping might be 50 – 300 µg/m³. The lower end reflects values seen in studies with low ventilation or disposable devices averaged over time, while the upper end reflects potential build-up from higher-intensity devices in a poorly ventilated space, drawing towards levels seen with tanks or nearer the source.⁷
• C_Nicotine: Based on the range observed across relevant studies ⁶, a plausible 1-hour average range is 1.0 – 4.0 µg/m³. This aligns with measurements using various devices under low-moderate ventilation.

3. Breathing Rate (BR): A standard breathing rate for a resting or lightly active adult bystander will be assumed. A typical value is 12 Liters per minute (L/min).²⁸

• BR = 12 L/min = 0.012 m³/min = 0.72 m³/hour.

4. Exposure Duration (T): As specified in the query, T = 1 hour.
5. Deposition Fraction (DF): For estimating the inhaled amount, we assume DF=1. This represents the total mass entering the bystander’s respiratory system. Actual deposition would be less than 100% and vary based on particle size and breathing patterns ³³, but quantifying this accurately for SHA in bystanders is beyond current data.

It is important to recognize that using a single average concentration value (C) over the hour simplifies the real-world dynamics. In a closed room with a constant source, concentrations will build up over the hour, particularly for gaseous components or non-evaporating particles.²⁸ The true time-averaged concentration for the 1-hour period might differ from a short-term measurement or a theoretical steady-state value. The selected concentration range attempts to account for potential build-up under low ventilation conditions.

VI. Estimated Inhaled Dose and Associated Uncertainties

Based on the concentration-based approach and the parameters defined above, the estimated inhaled dose for a bystander exposed to SHA for 1 hour in a closed 32 m³ room with constant vaping is calculated below.

A. Quantitative Estimate/Range

1. PM2.5 Inhaled Mass:

• Dose_PM2.5 = C_PM2.5 × BR × T
• Using C_PM2.5 range = 50 – 300 µg/m³
• Using BR = 0.72 m³/hr and T = 1 hr
• Lower Estimate: Dose_PM2.5 = 50 µg/m³ × 0.72 m³/hr × 1 hr = 36 µg
• Upper Estimate: Dose_PM2.5 = 300 µg/m³ × 0.72 m³/hr × 1 hr = 216 µg
• Estimated Range: The bystander is estimated to inhale between 36 and 216 micrograms (µg) of PM2.5 over the 1-hour period.

2. Nicotine Inhaled Mass:

• Dose_Nicotine = C_Nicotine × BR × T
• Using C_Nicotine range = 1.0 – 4.0 µg/m³
• Using BR = 0.72 m³/hr and T = 1 hr
• Lower Estimate: Dose_Nicotine = 1.0 µg/m³ × 0.72 m³/hr × 1 hr = 0.72 µg
• Upper Estimate: Dose_Nicotine = 4.0 µg/m³ × 0.72 m³/hr × 1 hr = 2.88 µg
• Estimated Range: The bystander is estimated to inhale between 0.72 and 2.88 micrograms (µg) of nicotine over the 1-hour period.

3. PG/VG Inhaled Mass: Data for PG/VG concentrations under these specific conditions are limited. Study ²⁴ provides 4-hour averages in a larger, better-ventilated room (PG up to ~10 µg/m³, VG up to ~32 µg/m³). Using a modeling approach based on exhaled amounts ³¹, estimated 1-hour PG concentration was ~1.19 mg/m³ (1190 µg/m³). Applying this concentration yields a potential inhaled PG dose of ~857 µg. Given the high uncertainty and reliance on modeling assumptions (like 10 puffs/min), a reliable estimate for PG/VG inhaled dose cannot be confidently provided based solely on the available experimental data synthesized here. However, it is likely that the inhaled mass of PG/VG would be substantially higher than that of nicotine, potentially exceeding the PM2.5 mass depending on aerosol composition.

B. Explicit Statement of Assumptions

The estimates above are based on the following key assumptions:

1. Room Conditions: Volume = 32 m³. “Closed” is interpreted as minimal ventilation, approximated by an Air Change Rate (ACH) of ≤ 1 hr⁻¹ (typical for infiltration in a sealed room). Standard room temperature and humidity are assumed.
2. Vaping Source: A single vaper using a modern e-cigarette device. “Constant vaping” is interpreted as a moderate-to-high intensity usage pattern sufficient to maintain the average concentrations (C_PM2.5 = 50-300 µg/m³, C_Nicotine = 1.0-4.0 µg/m³) over 1 hour. This implicitly assumes a certain number of puffs per hour and mass emitted per puff, characteristic of devices capable of producing these concentrations (potentially leaning towards tank systems or frequent use of disposables). Typical e-liquid properties (nicotine concentration, PG/VG ratio) are assumed unless linked to specific study data used for C.
3. Exhaled Aerosol: The chosen concentration ranges (C) reflect levels measured in environments with human vaping, thus accounting for typical retention by the vaper and exhalation into the room air.
4. Bystander: An adult bystander with a resting/light activity breathing rate (BR) of 12 L/min (0.72 m³/hr). The bystander is assumed to be located such that they are exposed to the average room concentration (i.e., not in the direct plume immediately after exhalation). The calculation estimates the inhaled dose (Deposition Fraction = 1).
5. Concentration Profile: The selected concentration ranges (C) are assumed to represent the average concentration experienced by the bystander over the entire 1-hour exposure period, accounting implicitly for some build-up in the closed room.

C. Discussion of Limitations and Uncertainties

This estimation is subject to significant limitations and uncertainties:

1. Data Scarcity and Extrapolation: No single study perfectly replicates the 32 m³, closed room, 1-hour constant vaping scenario. The concentration ranges (C) were synthesized by interpreting and extrapolating from studies with varying room sizes, ventilation rates (often higher than assumed here), measurement durations, and vaping protocols. This introduces considerable uncertainty.
2. Source Variability: This is arguably the largest source of uncertainty. The actual amount of PM2.5 and nicotine released into the room heavily depends on the specific device (disposable vs. tank, power settings), the e-liquid (nicotine strength, PG/VG ratio, flavors), and the user’s topography (puff frequency, duration, inhalation depth). The term “constant vaping” is ambiguous and was interpreted as moderate-to-high intensity; different interpretations would yield different results. The provided estimate range attempts to capture some of this variability but cannot encompass all possibilities.
3. Ventilation Uncertainty: The assumption of ACH ≤ 1 hr⁻¹ for a “closed room” is an approximation. Actual infiltration rates vary. Higher ventilation would reduce concentrations and the estimated dose.
4. Measurement Challenges: Variability exists in how PM2.5 and nicotine are measured across studies (instrumentation, sampling time, limits of quantification).³⁴ The rapid evaporation of SHA particles can complicate particle mass and number measurements.¹⁹
5. Model Simplification: The concentration-based model assumes uniform mixing within the room and uses a time-averaged concentration. It does not capture potential spatial gradients (higher exposure closer to the vaper ¹⁶) or the precise temporal dynamics of concentration build-up versus reaching a steady state within the hour. Deposition rates onto surfaces, which act as a removal mechanism, are also simplified or ignored (DF=1).
6. Bystander Variability: Individual breathing rates differ based on age, sex, activity level, and health status. The actual fraction of inhaled aerosol deposited in different parts of the bystander’s respiratory tract is complex and not precisely known for SHA.

Considering these factors, the provided dose estimates should be viewed as indicative ranges under plausible interpretations of the scenario, rather than precise predictions. The actual inhaled dose could be lower or higher depending on the specific details of the vaping activity and room conditions.

The estimated inhaled PM2.5 dose (36-216 µg) results from exposure to average concentrations (50-300 µg/m³) that significantly exceed typical ambient air quality guidelines (e.g., WHO 24-hr guideline of 15 µg/m³). This suggests that from a particulate matter perspective, the exposure scenario could be considered significant. Conversely, the estimated inhaled nicotine dose (0.72-2.88 µg) is substantially lower than what would be expected from similar exposure durations to significant levels of secondhand tobacco smoke, reflecting the lower airborne nicotine concentrations measured in SHA compared to SHS.⁵

VII. Conclusion

This report aimed to estimate the inhaled dose of secondhand vape aerosol (SHA) for a bystander in a closed 32 m³ room during one hour of constant vaping. Based on a synthesis of available literature measuring SHA components in controlled indoor environments and applying a standard concentration-based dose estimation approach, the following estimates were derived:

• Inhaled PM2.5: Approximately 36 to 216 micrograms (µg).
• Inhaled Nicotine: Approximately 0.72 to 2.88 micrograms (µg).
• Inhaled PG/VG: Difficult to quantify reliably from available experimental data, but likely substantial, potentially exceeding PM2.5 mass.

Key findings supporting this estimate include:

• SHA significantly increases indoor concentrations of PM2.5, often to levels exceeding ambient air quality guidelines, particularly under conditions of low ventilation and with higher-power devices.
• Airborne nicotine concentrations in SHA are measurable but are consistently found to be substantially lower (e.g., ~10 times lower in one direct comparison) than those in secondhand tobacco smoke under similar conditions.
• SHA also contains propylene glycol, vegetable glycerin, flavorings, and potentially harmful byproducts like aldehydes and trace metals, although quantifying bystander inhaled dose for these is more challenging due to data limitations and variability.

These estimates are subject to considerable uncertainty, primarily driven by:

• The high variability in emissions based on the specific vaping device, e-liquid, and user behavior (defining “constant vaping”).
• The precise ventilation characteristics of the “closed room.”
• The need to extrapolate from existing studies that may not perfectly match the scenario parameters.
• Simplifications inherent in the concentration-based dose model.

Despite these uncertainties, the analysis indicates that under the specified conditions of constant vaping in a small, closed room for one hour, a bystander is likely exposed to elevated levels of particulate matter, along with lower, but detectable, levels of nicotine and other aerosol constituents. While SHA is distinct from SHS, particularly regarding nicotine delivery to bystanders, the potential exposure to fine and ultrafine particles warrants consideration from an indoor air quality and public health perspective. Further research characterizing SHA concentrations and bystander exposure under well-defined, common real-world scenarios is needed to refine these estimates and better understand potential health implications.

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