Exhaled aerosols among PCR-confirmed SARS-CoV-2-infected children.

Background: Available data on aerosol emissions among children and adolescents during spontaneous breathing are limited. Our aim was to gain insight into the role of children in the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and whether aerosol measurements among children can be used to help detect so-called superspreaders-infected individuals with extremely high numbers of exhaled aerosol particles. Methods: In this prospective study, the aerosol concentrations of SARS-CoV-2 PCR-positive and SARS-CoV-2 PCR-negative children and adolescents (2-17 years) were investigated. All subjects were asked about their current health status and medical history. The exhaled aerosol particle counts of PCR-negative and PCR-positive subjects were measured using the Resp-Aer-Meter (Palas GmbH, Karlsruhe, Germany) and compared using linear regression. Results: A total of 250 children and adolescents were included in this study, 105 of whom were SARS-CoV-2 positive and 145 of whom were SARS-CoV-2 negative. The median age in both groups was 9 years (IQR 7-11 years). A total of 124 (49.6%) participants were female, and 126 (50.4%) participants were male. A total of 81.9% of the SARS-CoV-2-positive group had symptoms of viral infection. The median particle count of all individuals was 79.55 particles/liter (IQR 44.55-141.15). There was a tendency for older children to exhale more particles (1-5 years: 79.54 p/L; 6-11 years: 77.96 p/L; 12-17 years: 98.63 p/L). SARS-CoV-2 PCR status was not a bivariate predictor (t = 0.82, p = 0.415) of exhaled aerosol particle count; however, SARS-CoV-2 status was shown to be a significant predictor in a multiple regression model together with age, body mass index (BMI), COVID-19 vaccination, and past SARS-CoV-2 infection (t = 2.81, p = 0.005). COVID-19 vaccination status was a highly significant predictor of exhaled aerosol particles (p < .001). Conclusion: During SARS-CoV-2 infection, children and adolescents did not have elevated aerosol levels. In addition, no superspreaders were found.

Aerosol measurement identifies SARS-CoV 2 PCR positive adults compared with healthy controls.

BACKGROUND: SARS-CoV-2 is spread primarily through droplets and aerosols. Exhaled aerosols are generated in the upper airways through shear stress and in the lung periphery by 'reopening of collapsed airways'. Aerosol measuring may detect highly contagious individuals ("super spreaders or super-emitters") and discriminate between SARS-CoV-2 infected and non-infected individuals. This is the first study comparing exhaled aerosols in SARS-CoV-2 infected individuals and healthy controls. DESIGN: A prospective observational cohort study in 288 adults, comprising 64 patients testing positive by SARS CoV-2 PCR before enrollment, and 224 healthy adults testing negative (matched control sample) at the University Hospital Frankfurt, Germany, from February to June 2021. Study objective was to evaluate the concentration of exhaled aerosols during physiologic breathing in SARS-CoV-2 PCR-positive and -negative subjects. Secondary outcome measures included correlation of aerosol concentration to SARS-CoV-2 PCR results, change in aerosol concentration due to confounders, and correlation between clinical symptoms and aerosol. RESULTS: There was a highly significant difference in respiratory aerosol concentrations between SARS-CoV-2 PCR-positive (median 1490.5/L) and -negative subjects (median 252.0/L; p < 0.0001). There were no significant differences due to age, sex, smoking status, or body mass index. ROC analysis showed an AUC of 0.8918. CONCLUSIONS: Measurements of respiratory aerosols were significantly elevated in SARS-CoV-2 positive individuals, which helps to understand the spread and course of respiratory viral infections, as well as the detection of highly infectious individuals.

[Interdisciplinary Perspectives on the Role of Aerosol Transmission in SARS-CoV-2 Infections]. / Interdisziplinäre Perspektiven zur Bedeutung der Aerosolübertragung für das Infektionsgeschehen von SARS-CoV-2.

The relevance of aerosols for the transmission of the Severe Acute Respiratory Syndrome Coronavirus Type 2 (SARS-CoV-2) is still debated. However, over time, in addition to distancing and hygiene rules, aerosol physics-based measures such as wearing face masks and ventilating indoor spaces were found to be efficient in reducing infections. In an interdisciplinary workshop "Aerosol & SARS-CoV-2" of the Association for Aerosol Research (GAeF) in cooperation with the German Society for Pneumology and Respiratory Medicine (DGP), the Professional Association of General Air Technology of the VDMA, the German Society for Virology (GfV), the Health Technology Society (GG) and the International Society for Aerosols in Medicine (ISAM) under the auspices of the Robert Koch Institute (RKI) in March 2021, the need for research and coordination on this topic was addressed. Fundamental findings from the various disciplines as well as interdisciplinary perspectives on aerosol transmission of SARS-CoV-2 and infection mitigation measures are summarized here. Finally, open research questions and needs are presented.

COVID-19 symptoms are reduced by targeted hydration of the nose, larynx and trachea.

Dehydration of the upper airways increases risks of respiratory diseases from COVID-19 to asthma and COPD. We find in human volunteer studies involving 464 human subjects in Germany, the US, and India that respiratory droplet generation increases by up to 4 orders of magnitude in dehydration-associated states of advanced age (n = 357), elevated BMI-age (n = 148), strenuous exercise (n = 20) and SARS-CoV-2 infection (n = 87), and falls with hydration of the nose, larynx and trachea by calcium-rich hypertonic salts. We also find in a protocol of exercise-induced airway dehydration that hydration of the airways by calcium-rich salts increases oxygenation relative to a non-treatment control (P < 0.05). In a random control study of COVID-19 positive subjects (n = 40), thrice-a-day delivery of the calcium-rich hypertonic salts (active) suppressed respiratory droplet generation by 51% ± 11% and increased oxygen saturation over three days of treatment by 48.08% ± 9.61% (P < 0.001), while no changes were observed in the nasal-saline control group. Self-reported symptoms significantly declined in the active group and did not decline in the control group. Hydration of the upper airways appears promising as a non-drug approach for reducing risks of respiratory diseases such as COVID-19.

Regional Lung Deposition: In Vivo Data.

The method section of this chapter on in vivo regional lung deposition highlights a nonradioactive method to measure regional deposition, which uses a photometer to quantify inhaled and exhaled particles and in that way is able to estimate the lung region from which the particles are exhaled and to what amount. The radioactive methods cover the measurement of clearance of the deposited particles as well as different imaging techniques to determine regional deposition. The result section reviews in vivo trials in human subjects. It also addresses different parameters that influence the regional deposition in the lungs: particle size, inhalation maneuver, carrier gas, disease, and inhalation device. All of these factors can affect regional deposition significantly. By choosing specific values of these parameters, it should be feasible to target different regions of the lungs for the therapy of different diseases.

Breathing Is Enough: For the Spread of Influenza Virus and SARS-CoV-2 by Breathing Only.

Background: The transmission of respiratory viruses such as influenza and corona viruses from one person to another is still not fully understood. Methods: A literature search showed that there is a strong scientific rationale and evidence that viruses are very efficiently spread through aerosols by the patient's breathing only. It is not necessary for the patient to cough or sneeze. Results: The exhaled aerosol particles are generated by normal breathing in the deep lung through reopening of collapsed small airways during inspiration. These mucus/surfactant aerosols (size range between 0.2 and 0.6 µm) can transport viruses out of the lungs of patients and be present in the room air for hours. Conclusion: These aerosol particles are difficult to filter out of the air; because of their physical properties, new strategies must be developed to protect people from these virus aerosols.

Targeting intracellular signaling as an antiviral strategy: aerosolized LASAG for the treatment of influenza in hospitalized patients.

Influenza has been a long-running health problem and novel antiviral drugs are urgently needed. In pre-clinical studies, we demonstrated broad antiviral activity of D, L-lysine-acetylsalicylate glycine (LASAG) against influenza virus (IV) in cell culture and protection against lethal challenge in mice. LASAG is a compound with a new antiviral mode of action. It inhibits the NF-κB signal transduction module that is essential for IV replication. Our goal was to determine whether aerosolized LASAG would also show a therapeutic benefit in hospitalized patients suffering from severe influenza. The primary endpoint was time to alleviation of clinical influenza symptoms. The primary analysis was based on the modified intention-to-treat (MITT) population. This included all patients with confirmed influenza virus infection who received at least one treatment. The per protocol (PP) analysis set included all subjects from the MITT population who underwent at least 13 inhalations. In the MITT group, 48 (41.7%) participants (29 LASAG; 19 placebo) had severe influenza. The mean time to symptom alleviation was 56.2 h in the placebo group and 43.0 h in the LASAG group. The PP set consisted of 41 patients (24 LASAG; 17 placebo). The mean time to symptom alleviation in the LASAG group (38.3 h; P = 0.0365) was lower than that in the placebo group (56.2 h). In conclusion, LASAG improved the time to alleviation of influenza symptoms in hospitalized patients. The present phase II proof-of-concept (PoC) study demonstrates that targeting an intra-cellular signaling pathway using aerosolized LASAG improves the time to symptom alleviation compared to standard treatment.

Pharmacodynamics, Pharmacokinetics, and Antiviral Activity of BAY 81-8781, a Novel NF-κB Inhibiting Anti-influenza Drug.

Influenza is a respiratory disease that causes annual epidemics. Antiviral treatment options targeting the virus exist, but their efficiency is limited and influenza virus strains easily develop resistance. Thus, new treatment strategies are urgently needed. In the present study, we investigated the anti-influenza virus properties of D,L-lysine acetylsalicylate ⋅ glycine (BAY 81-8781; LASAG) that is approved as Aspirin i.v. for intravenous application. Instead of targeting the virus directly BAY 81-8781 inhibits the activation of the NF-κB pathway, which is required for efficient influenza virus propagation. Using highly pathogenic avian influenza virus strains we could demonstrate that BAY 81-8781 was able to control influenza virus infection in vitro. In the mouse infection model, inhalation of BAY 81-8781 resulted in reduced lung virus titers and protection of mice from lethal infection. Pharmacological studies demonstrated that the oral route of administration was not suitable to reach the sufficient concentrations of BAY 81-8781 for a successful antiviral effect in the lung. BAY 81-8781 treatment of mice infected with influenza virus started as late as 48 h after infection was still effective in protecting 50% of the animals from death. In summary, the data represent a successful proof of the novel innovative antiviral concept of targeting a host cell signaling pathway that is required for viral propagation instead of viral structures.

Patient Focus and Regulatory Considerations for Inhalation Device Design: Report from the 2015 IPAC-RS/ISAM Workshop.

This article reports on discussions at the 2015 workshop cosponsored by the International Pharmaceutical Aerosol Consortium on Regulation and Science (IPAC-RS) and the International Society for Aerosols in Medicine (ISAM), entitled "Regulatory and Patient Considerations for Inhalation Device Design, Development and Use." Key topics addressed at the workshop and presented here include patient-focused device design for orally inhaled products (OIPs), instructions for use (IFU), human factors, regulatory considerations in the United States and Europe, development of generic inhalers, quality-by-design, and change management of OIP devices. Workshop participants also identified several areas for further consideration and emphasized the need for increased focus on the patient to create therapeutic products (inclusive of device design, IFU, education, training) that support adherence with an individual patient's treatment regimen. Advances in patient-centric product development will require engagement and collaboration by industry, regulators, patients, physicians, and other stakeholders. The article includes summaries of presented talks as well as of panel and audience discussions.

Delivery of Alpha-1 Antitrypsin to Airways.

Treatment with exogenous alpha-1 antitrypsin (AAT), a potent serine protease inhibitor, was developed originally for chronic obstructive pulmonary disease associated with AAT deficiency; however, other lung conditions involving neutrophilic inflammation and proteolytic tissue injury related to neutrophil elastase and other serine proteases may also be considered for AAT therapy. These conditions include bronchiectasis caused by primary ciliary dyskinesia, cystic fibrosis, and other diseases associated with an increased free elastase activity in the airways. Inhaled AAT may be a viable option to counteract proteolytic tissue damage. This form of treatment requires efficient drug delivery to the targeted pulmonary compartment. Aerosol technology meeting this requirement is currently available and offers an alternative therapeutic approach to systemic AAT administration. To date, early studies in humans have shown biochemical efficacy and have established the safety of inhaled AAT. However, to bring aerosol AAT therapy to patients, large phase 3 protocols in carefully selected patient populations (i.e., subgroups of patients with AAT deficiency, cystic fibrosis, or other lung diseases with bronchiectasis) will be needed with clinical end points in addition to the measurement of proteolytic activity in the airway. The outcomes likely will have to include lung function, lung structure assessed by computed tomography imaging, disease exacerbations, health status, and mortality.

Aerosolized GLP-1 for treatment of diabetes mellitus and irritable bowel syndrome.

Diabetes is a global burden and the prevalence of the disease, in particular diabetes mellitus type 2 is rapidly increasing worldwide. After introduction of insulin into clinical therapy about 90 years ago a major number of pharmaceuticals has been developed for treatment of diabetes mellitus type 2. One of these, the incretin glucagon-like peptide 1 (GLP-1), like insulin, needs subcutaneous administration causing inconvenience to patients. However, administration of GLP-1 plays also a role for treatment of irritable bowel syndrome (IBS). To improve patient convenience inhaled insulin (Exubera(®)) was developed and approved but failed market acceptance some years ago. Recently, another inhalative insulin (Afrezza(®)) received market approval and GLP-1 may serve as another candidate drug for inhalative administration. This review analyzes the current literature investigating alternative administration of GLP-1 and GLP-1 analogs focusing on inhalation. Several formulations for inhalative administration of GLP-1 and analogs were investigated in animal studies, whereas there are only few clinical data. However, feasibility of GLP-1 inhalation has been shown and should be further investigated as such type of drug administration may serve for improvement of therapy in patients with diabetes mellitus or irritable bowel syndrome.

Nebulised budesonide using a novel device in patients with oral steroid-dependent asthma.

This phase 2/3 randomised, parallel-group, placebo-controlled trial investigated oral corticosteroid (OCS)-sparing efficacy, safety and tolerability of nebulised budesonide (Bud) administered with a novel computer-controlled, compressor-driven inhalation system (AKITA) as add-on therapy to Global Initiative for Asthma step 5. Patients (18-65 years) with OCS-dependent asthma were randomised (2:1:1:1) to receive 18-week, twice-daily, double-blind treatment with AKITA inhaled corticosteroid (AICS)-Bud 1 mg, AICS-Bud 0.5 mg, AICS-placebo or open-label Bud 1 mg administered by conventional nebuliser (CN-Bud). OCS doses were tapered until week 14. 199 patients started treatment. More AICS-Bud 1 mg (80.0%) than placebo-treated (62.5%) patients had daily OCS doses reduced ≥50%, with clinical stability to week 18 (one-sided p=0.02; treatment difference: 17.5% (95% CI 0.1-34.9%), two-sided p=0.04). Mean±sd forced expiratory volume in 1 s improved (from baseline to week 18) for AICS-Bud 1 mg (239±460 mL, p<0.001) and AICS-Bud 0.5 mg (126±345 mL, p=0.01) but not placebo (93±419 mL, p=0.36) or CN-Bud (137±459 mL, p=0.18). Fewer AICS-Bud 1 mg-treated patients experienced asthma exacerbations (7.5%) compared with placebo (17.5%) or CN-Bud (22.5%). All treatments were well tolerated. Budesonide applied with AKITA allowed significant meaningful OCS reduction in OCS-dependent asthma patients while improving pulmonary function and maintaining exacerbation control.

Bioequivalence of inhaled drugs: fundamentals, challenges and perspectives.

Interest in bioequivalence (BE) of inhaled drugs derives largely from the desire to offer generic substitutes to successful drug products. The complexity of aerosol dosage forms renders them difficult to mimic and raises questions regarding definitions of similarities and those properties that must be controlled to guarantee both the quality and the efficacy of the product. Despite a high level of enthusiasm to identify and control desirable properties there is no clear guidance, regulatory or scientific, for the variety of aerosol dosage forms, on practical measures of BE from which products can be developed. As more data on the pharmaceutical and clinical relevance of various techniques, as described in this review, become available, it is likely that a path to the demonstration of BE will become evident. In the meantime, debate on this topic will continue.

Effects of formoterol and tiotropium bromide on mucus clearance in patients with COPD.

BACKGROUND: Lung mucociliary clearance is impaired in patients with chronic obstructive pulmonary disease (COPD). Treatment guidelines recommend that patients with COPD receive maintenance therapy with long-acting beta-agonists and anticholinergic agents. METHODS: Twenty-four patients with mild to moderate COPD received formoterol (12 µg, twice daily from Turbuhaler® dry powder inhaler (DPI)) or tiotropium (18 µg, once daily from Handihaler® DPI) for 14 days. They also received single doses of formoterol, tiotropium, salbutamol (200 µg) and placebo. A radioaerosol technique was used to assess the effects on mucus clearance of 14 days treatment with formoterol or tiotropium, as well as single doses of these drugs. RESULTS: The 4 h whole lung retention of radioaerosol was significantly higher after 14 days treatment with tiotropium (P = 0.016), but not after 14 days treatment with formoterol. However, patients bronchodilated after 14 days treatment with both drugs, so that the deposited radioaerosol may have had an increased distance to travel in order to be cleared by mucociliary action. A single dose of formoterol enhanced radioaerosol clearance significantly compared to other single dose treatments (P < 0.05). CONCLUSION: Formoterol (12 µg) enhances mucus clearance in patients with mild to moderate COPD when given as a single dose, and may do so when given for 14 days. Studies of longer duration would be needed in order to assess the effects of the study drugs on mucus clearance when they are used for long-term maintenance therapy.

Deposition, imaging, and clearance: what remains to be done?

Deposition and clearance studies are used during product development and in fundamental research. These studies mostly involve radionuclide imaging, but pharmacokinetic methods are also used to assess the amount of drug absorbed through the lungs, which is closely related to lung deposition. Radionuclide imaging may be two-dimensional (gamma scintigraphy or planar imaging), or three-dimensional (single photon emission computed tomography and positron emission tomography). In October 2009, a group of scientists met at the "Thousand Years of Pharmaceutical Aerosols" conference in Reykjavik, Iceland, to discuss future research in key areas of pulmonary drug delivery. This article reports the session on "Deposition, imaging and clearance." The objective was partly to review our current understanding, but more importantly to assess "what remains to be done?" A need to standardize methodology and provide a regulatory framework by which data from radionuclide imaging methods could be compared between centers and used in the drug approval process was recognized. There is also a requirement for novel radiolabeling methods that are more representative of production processes for dry powder inhalers and pressurized metered dose inhalers. A need was identified for studies to aid our understanding of the relationship between clinical effects and regional deposition patterns of inhaled drugs. A robust methodology to assess clearance from small conducting airways should be developed, as a potential biomarker for therapies in cystic fibrosis and other diseases. The mechanisms by which inhaled nanoparticles are removed from the lungs, and the factors on which their removal depends, require further investigation. Last, and by no means least, we need a better understanding of patient-related factors, including how to reduce the variability in pulmonary drug delivery, in order to improve the precision of deposition and clearance measurements.

Mucociliary and cough clearance as a biomarker for therapeutic development.

A workshop/symposium on "Mucociliary and Cough Clearance (MCC/CC) as a Biomarker for Therapeutic Development" was held on October 21­22, 2008, in Research Triangle Park, NC, to discuss the methods for measurement of MCC/CC and how they may be optimized for assessing new therapies designed to improve clearance of airway secretions from the lungs. The utility of MCC/CC as a biomarker for disease progression and therapeutic intervention is gaining increased recognition as a valuable tool in the clinical research community. A number of investigators currently active in using MCC/CC for diagnostic or therapeutic evaluation presented details of their methodologies. Attendees participating in the workshop discussions included those interested in the physiology of MCC/CC, some of who use in vitro or animal methods for its study, pharmaceutical companies developing muco-active therapies, and many who were interested in establishing the methods in their own clinical laboratory. This review article summarizes the presentations for the in vivo human MCC/CC methods and the discussions both at and subsequent to the workshop between the authors to move forward on a number of questions raised at the workshop.

Safety and tolerability of inhaled heparin in idiopathic pulmonary fibrosis.

BACKGROUND: Abnormalities in alveolar coagulation occur in idiopathic pulmonary fibrosis (IPF). Anticoagulants attenuate bleomycin-induced lung fibrosis in animals. In this study, we first examined the pharmacokinetics of inhaled heparin in healthy subjects. Second, we investigated the safety and tolerability of heparin inhalation in IPF patients. METHODS: Coagulation assays were performed in blood and bronchoalveolar lavage fluid samples from 19 healthy volunteers after inhalation of increasing amounts of unfractionated heparin. The acute effects of heparin inhalation on lung function and exercise capacity and the safety and tolerability of chronic heparin inhalation for 28 days were assessed in 20 IPF patients in an open-label exploratory pilot study. RESULTS: In healthy subjects, inhalation of 150,000 IU heparin ("filled dose") significantly increased the partial thromboplastin time and anti-factor Xa activity in blood samples indicating the threshold dose. The local alveolar anticoagulant effect was detectable up to 72 h, and the alveolar half-life was estimated at 28 h. In IPF-patients, no acute deleterious effects on pulmonary function, gas exchange, or exercise capacity were noted after inhalation of the threshold dose. During chronic treatment, where one-fourth of the threshold dose was inhaled every 12 h for 28 days to obtain a steady-state anticoagulant activity in the alveolar space approximating the anticoagulant activity observed after threshold dose inhalation, no heparin-related side effects, such as hemoptysis or heparin-induced antibodies and thrombocytopenia, were detected in any patient, and median lung function values, exercise capacity, and quality of life scores appeared largely unaltered. Three adverse and one serious adverse events were noted; however, the relation of these events to the heparin inhalation was assessed as "unlikely" or "no relation" in each case. CONCLUSIONS: Inhaled heparin appears to be safe and well tolerated in IPF patients. Future clinical trials are required to demonstrate the long-term safety and efficacy of inhaled heparin in IPF.

Left-to-right asymmetry of aerosol deposition after shallow bolus inhalation depends on lung ventilation.

BACKGROUND: After shallow bolus inhalation of radiolabeled aerosols, gamma camera imaging has shown a left-right asymmetry, with a higher fraction of deposited particles in the left lung. It was not clear, however, whether this phenomenon was an effect of asymmetry in lung ventilation or aerosol deposition efficiency. METHODS: Lung ventilation and aerosol deposition was studied after shallow bolus inhalation and gamma camera imaging in nine healthy nonsmokers and 10 asymptomatic smokers. A 100-mL (81m)Kr-gas boli were administered within the Fowler and within the phase-1 dead space, respectively. In addition, 1-L full breaths of 81m-Kr-gas were inhaled. For aerosol deposition subjects inhaled 100-mL boli of 100-nm diameter radiolabeled carbon particles with shallow and deep penetration. Left-to-right (L/R) and central-to-peripheral (C/P) activity distribution of the lung was analyzed. RESULTS: None of the parameters analyzed were significantly different between nonsmokers and smokers. The full-breath 81m-Kr-gas inhalation revealed a similar activity distribution over the left and right lungs, according to their respective volumes (L/R ratio = 0.84 +/- 0.04; mean +/- SE). In contrast, the shallow bolus inhalation of 81m-Kr-gas to the phase-1 dead space revealed more activity in the left lung (L/R ratio = 1.49 +/- 0.15, normalized to full-breath Kr-gas L/R). This same left-right asymmetry was observed for the aerosol after shallow bolus inhalation (L/R ratio = 1.69 +/- 0.15), and there was no significant difference between Kr-gas and aerosol L/R ratio. C/P activity ratios of bolus inhalation to the phase-1 dead space were 1.71 +/- 0.19 and 1.79 +/- 0.15 (normalized to full-breath Kr-gas C/P) for gas and aerosol, respectively, and correlated with the L/R ratios. CONCLUSIONS: The data show that the asymmetry in shallow aerosol bolus deposition is primarily determined by lung ventilation. The reason for this asymmetry is unclear.

Deposition, retention, and translocation of ultrafine particles from the central airways and lung periphery.

RATIONALE: Little is known about clearance of ultrafine carbon particles from the different regions of the human lung. These particles may accumulate and present a health hazard because of their high surface area. OBJECTIVES: Technetium Tc 99m ((99m)Tc)-radiolabeled 100-nm-diameter carbon particles were inhaled by healthy nonsmokers, asymptomatic smokers, and by patients with chronic obstructive pulmonary disease (COPD). METHODS: Using a bolus inhalation technique, particle deposition was targeted either to the airways or to the lung periphery, and retention, clearance, and translocation were measured using retained radiotracer imaging. MEASUREMENTS AND MAIN RESULTS: In vitro studies revealed that mean leaching of soluble (99m)Tc-radiotracer from the carbon particles was 4.1 (2.6 [SD]) % after 24 hours. Cumulative (99m)Tc activity in urine at 24 hours was 1.1 (1.3) % of activity deposited in the lungs. In the lung periphery, particle retention was not affected by smoking or pulmonary disease; retention was 96 (3) % after 24 hours. The small amount of clearance could be attributed to leaching of the (99m)Tc label, suggesting negligible particle clearance. In healthy nonsmokers, retention of particles targeted to the airways was 89 (6) and 75 (10) % after 1.5 and 24 hours, respectively. Radiolabel activity did not accumulate in the liver. CONCLUSIONS: Within the limits of detection of our experimental system, most inhaled ultrafine carbon particles are retained in the lung periphery and in the conducting airways without substantial systemic translocation or accumulation in the liver at 48 hours. Repeated exposure may result in significant pulmonary accumulation of ultrafine particles.