28 research outputs found
On modeling airborne infection risk
Airborne infection risk analysis is usually performed for enclosed spaces
where susceptible individuals are exposed to infectious airborne respiratory
droplets by inhalation. It is usually based on exponential, dose-response
models of which a widely used variant is the Wells-Riley (WR) model. We employ
a population-based Susceptible-Exposed-Droplet-Infected-Recovered (SEDIR) model
to revisit the infection-risk estimate at the population level during an
epidemic. We demonstrate the link between epidemiological models and the WR
model, including its Gammaitoni-Nucci (GN) generalization. This connection
shows how infection quanta are related to the number of infectious airborne
droplets. For long latent periods, the SEDIR model reduces to the GN model with
parameters that depend on biological properties of the pathogen (size-dependent
pathogen droplet concentration, infection probability of a deposited infectious
droplet), physical droplet properties (lung-deposition probability), and
individual behavioral properties (exposure time). In two scenarios we calculate
the probability of infection during the epidemic. The WR and GN limits of the
SEDIR model reproduce accurately the SEDIR-calculated infection risk.Comment: 14 pages, 3 figure
On the friction coefficient of straight-chain aggregates
A methodology to calculate the friction coefficient of an aggregate in the
continuum regime is proposed. The friction coefficient and the monomer
shielding factors, aggregate-average or individual, are related to the
molecule-aggregate collision rate that is obtained from the molecular diffusion
equation with an absorbing boundary condition on the aggregate surface.
Calculated friction coefficients of straight chains are in very good agreement
with previous results, suggesting that the friction coefficients may be
accurately calculated from the product of the collision rate and an average
momentum transfer,the latter being independent of aggregate morphology.
Langevin-dynamics simulations show that the diffusive motion of straight-chain
aggregates may be described either by a monomer-dependent or an
aggregate-average random force, if the shielding factors are appropriately
chosen.Comment: 22 pages, 6 figures, revised version. To appear in the Journal of
Colloid and Interface Scienc
Lockdown Measures and their Impact on Single- and Two-age-structured Epidemic Model for the COVID-19 Outbreak in Mexico
The role of lockdown measures in mitigating COVID-19 in Mexico is investigated using a comprehensive
nonlinear ODE model. The model includes both asymptomatic and presymptomatic populations with the latter
leading to sickness (with recovery, hospitalization and death possibilities). We consider the situation involving
the imposed application of partial social distancing measures in the time series of interest and find optimal parametric fits to the time series of deaths (only), as well as to that of deaths and cumulative infections. We discuss
the merits and disadvantages of each approach, we interpret the parameters of the model and assess the realistic
nature of the parameters resulting from the optimization procedure. Importantly, we explore a model involving
two sub-populations (younger and older than a specific age), to more accurately reflect the observed impact as
concerns symptoms and behavior in different age groups. For definitiveness and to separate people that are (typically) in the active workforce, our partition of population is with respect to members younger vs. older than the
age of 65. The basic reproductive number of the model is computed for both the single- and the two-population
variant. Finally, we consider what would be the impact on the number of deaths and cumulative infections upon
imposition of partial lockdown (involving only the older population) and full lockdown (involving the entire
population)
Number Concentrations and Modal Structure of Indoor/Outdoor Fine Particles in Four European Cities
Indoor/outdoor aerosol size distribution was measured in four European cities (Oslo-Norway, Prague-Czech Republic, Milan-Italy and Athens-Greece) during 2002 in order to examine the differences in the characteristics of the indoor/outdoor modal structure and to evaluate the effect of indoor sources to the aerosol size distributions. All the measurement sites were naturally ventilated and were occupied during the campaigns by permanent residents or for certain time periods by the technical staff responsible for the instrumentation. Outdoor particle number (PN) concentrations presented the higher values in Milan and Athens (median values 1.4 x 10(4) # cm(-3) and 2.9 x 10(4) # cm(-3) respectively) as a result of elevated outdoor emissions and led to correspondingly higher indoor values compared to Oslo and Prague. In absence of indoor activities, the indoor concentrations followed the fluctuations of the outdoor concentrations in all the measurement sites. Indoor activities (cooking, smoking, etc.) resulted in elevated indoor PN concentrations (maximum values ranging between 1.7 x 10(5) # cm(-3) and 3.2 x 10(5) # cm(-3)) and to I/O ratios higher than one. The I/O ratios were size dependant and for periods without indoor activities, they presented the lowest values for particles <50 nm (0.51 +/- 0.15) and the ratios increased with fine particle size (0.79 +/- 0.12 for particles between 100-200 nm). The analysis of the modal structure showed that the indoor aerosol size distribution characteristics differ from the outdoors under the effect of indoor sources. The percentage of unimodal size distributions increased during indoor emissions, compared to periods without indoor sources, along with the number concentration of Aitken mode particles, indicating emissions in specific size ranges according to the type of the indoor source.Peer reviewe
Dynamics of infectious disease transmission by inhalable respiratory droplets
Transmission of respiratory infectious diseases in humans, for instance influenza, occurs by several modes. Respiratory droplets provide a vector of transmission of an infectious pathogen that may contribute to different transmission modes. An epidemiological model incorporating the dynamics of inhalable respiratory droplets is developed to assess their relevance in the infectious process. Inhalable respiratory droplets are divided into respirable droplets, with droplet diameter less than 10 µm, and inspirable droplets, with diameter in the range 10–100 µm: both droplet classes may be inhaled or settle. Droplet dynamics is determined by their physical properties (size), whereas population dynamics is determined by, among other parameters, the pathogen infectivity and the host contact rates. Three model influenza epidemic scenarios, mediated by different airborne or settled droplet classes, are analysed. The scenarios are distinguished by the characteristic times associated with breathing at contact and with hand-to-face contact. The scenarios suggest that airborne transmission, mediated by respirable droplets, provides the dominant transmission mode in middle and long-term epidemics, whereas inspirable droplets, be they airborne or settled, characterize short-term epidemics with high attack rates. The model neglects close-contact transmission by droplet sprays (direct projection onto facial mucous membranes), retaining close-contact transmission by inspirable droplets