sars-cov-2 in droplets and aerosols
TRANSCRIPT
S A R S - Co V - 2 i n
D r o p l e t s a n d A e ros o l s
LINSEY C. MARR
CHARLES P. LUNSFORD PROFESSOR
CIVIL AND ENVIRONMENTAL ENGINEERING
VIRGINIA TECH
7 JULY 2020
Topics
1. Transmission modes
2. Size distributions
3. Evaporation and transport
4. Air sampling results and challenges
Linsey Marr, Virginia Tech, July 2020 https://phil.cdc.gov/Details.aspx?pid=11160
2
The origin of the 5-10 µm cutoff is not clear. This cutoff is not supported by modern aerosol science. This distinction has hampered our understanding of transmission.
Traditional Modes of Transmission
http://www.phac-aspc.gc.ca/cpip-pclcpi/annf/v2-eng.php
direct contact indirect contact
large droplets airborne/aerosols
Linsey Marr, Virginia Tech, July 2020 3
Defined as >5 m and happening at close-range only (<2 m)
Defined as <5 m and happening mainly at long-distance (>2 m)
Multiple Modes at Close Contact
Linsey Marr, Virginia Tech, July 2020 4 https://www.cidrap.umn.edu/news-perspective/2020/03/commentary-covid-19-transmission-messages-should-hinge-science
droplet spray AND
inhalation of aerosols
Aerosols Dilute and Spread
Linsey Marr, Virginia Tech, July 2020 5 https://www.cidrap.umn.edu/news-perspective/2020/03/commentary-covid-19-transmission-messages-should-hinge-science
Aerosols Can Build Up Indoors
Linsey Marr, Virginia Tech, July 2020 6 https://www.cidrap.umn.edu/news-perspective/2020/03/commentary-covid-19-transmission-messages-should-hinge-science
ventilation to eliminate buildup of aerosols
Face Coverings for Source Control
Linsey Marr, Virginia Tech, July 2020 7 https://www.cidrap.umn.edu/news-perspective/2020/03/commentary-covid-19-transmission-messages-should-hinge-science
Droplets vs. Aerosols
• Distinguishing feature should be route of exposure
Linsey Marr, Virginia Tech, July 2020 8
CONTACT with DROPLETS landing on the body
INHALATION of AEROSOLS into the respiratory system
Linsey Marr, Virginia Tech, July 2020 9
Transmission by inhalation of aerosols can happen at short and long range.
Why is size so important?
Virus Size
Linsey Marr, Virginia Tech, July 2020 10
influenza 0.1 m
SARS-CoV-2 0.12 m
rhinovirus 0.03 m
https://www.cdc.gov/flu/resource-center/freeresources/graphics/images.htm, http://solutionsdesignedforhealthcare.com/rhinovirus, https://phil.cdc.gov/Details.aspx?pid=23312, https://pdb101.rcsb.org/motm/132
adenovirus 0.1 m
Size Matters
• Airborne viruses are not naked!
• An aerosol is just a very small droplet
• Size determines • How far they can travel in air • Where they deposit in the respiratory system
0.1 m
0.5 m
(0.2-100 m)
respiratory fluid
Linsey Marr, Virginia Tech, July 2020 11
Humidity Affects Size and Chemistry
Linsey Marr, Virginia Tech, July 2020 14
Low RH Very high RH Medium RH
salt
protein
Settling Velocity and Time
Droplets smaller than 10 m can remain suspended for many minutes to hours
Linsey Marr, Virginia Tech, July 2020 15
𝑠𝑒𝑡𝑡𝑖𝑛𝑔 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑣 = 𝑔𝐷𝑝
2𝜌𝑝
18𝜇
particle diameter particle density
dynamic viscosity of air
5 µm
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 20 40 60 80
Hei
ght
(m)
Distance (m)
Low indoor air velocity (5 cm/s)
5 um
10 um
20 um
30 um
Droplets Can Travel More Than 2 m
Linsey Marr, Virginia Tech, July 2020 16
Nazaroff, 2020, personal communication
trajectory of 5 m droplet
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 50 100 150 200 250 300
Hei
ght
(m)
Distance (m)
High indoor air velocity (20 cm/s)
5 um
10 um
20 um
30 um
Position of droplets released from a height of 1 m
Linsey Marr, Virginia Tech, July 2020 17
Viruses in droplets/aerosols >5-10 µm remain in air for many minutes and can travel farther than 1-2 m. What do we know about SARS-CoV-2 in droplets/aerosols?
Virus Detection Methods
1. Total virus • Number of genome copies
(GC) determined by molecular techniques (quantitative polymerase chain reaction, qPCR)
• Reflects number of viruses with intact DNA or RNA
• Does NOT indicate whether virus is infectious or not
Linsey Marr, Virginia Tech, July 2020 18 https://www.cdc.gov/flu/resource-center/freeresources/graphics/images.htm
RNA is wrapped around the ribonucleoprotein
Virus Detection Methods
2. Infectious virus • Number of viruses that are able
to infect cells determined by culture (growing)
• PFU = plaque forming units, number of viruses capable of forming plaques on host cells, focus forming units (FFU) are related
• TCID50 = median tissue culture infectious dose, concentration at which half of cells are infected after being exposed to the sample
Linsey Marr, Virginia Tech, July 2020 19 https://phil.cdc.gov/Details.aspx?pid=23213
Linsey Marr, Virginia Tech, July 2020 20 Ong et al., 2020, JAMA, https://jamanetwork.com/journals/jama/fullarticle/2762692
Samples in patient A’s and B’s rooms were negative after cleaning. These results are for patient C before cleaning.
table, bed rail, locker, chair, light switches, stethoscope, sink, floor, window, door and handle, toilet, air outlet fan
control panel on bed, call bell, rail in bathroom, outer rim of sink in bathroom, all anteroom, air
negative
positive
Linsey Marr, Virginia Tech, July 2020 21 Chia et al., 2020, https://www.nature.com/articles/ s41467-020-16670-2; Santarpia et al., 2020, https://www.medrxiv.org/content/10.1101/2020.03.23.20039446v2
Airborne Viral RNA in Hospitals
I estimate a viral RNA emission rate of 10,000 genome copies per minute in “small” droplets.
Liu et al., 2020, Nature, https://www.nature.com/articles/s41586-020-2271-3
SARS-CoV-2 Size Distributions
Linsey Marr, Virginia Tech, July 2020 22
SARS-CoV-2 Survival in Aerosols
Linsey Marr, Virginia Tech, July 2020 23 van Doremalen et al., 2020, NEJM, https://www.nejm.org/doi/full/10.1056/NEJMc2004973
Half-life is 1.1 hours in culture medium at 65% RH.
SARS-CoV-2 Survival in Aerosols
Linsey Marr, Virginia Tech, July 2020 24 Fears et al., 2020, https://wwwnc.cdc.gov/eid/article/26/9/20-1806_article
Virus survives 16 hours in aerosols in culture medium at 53% RH
Sampling Challenges
• Low concentrations of ~103 viral RNA copies/m3
• Probably need ~104 copies to get 1 PFU (10 m3 air)
• Ideally want size separation
Linsey Marr, Virginia Tech, July 2020 25 Prussin II et al., 2014, https://onlinelibrary.wiley.com/ doi/abs/10.1111/1574-6968.12487; https://www.vox.com/science-and-health/2020/3/11/21173187/coronavirus-covid-19-hand-washing-sanitizer-compared-soap-is-dope
Must must elute and extract large filter
Low flow rates require 20-40 hr of sampling
Loss of infectivity during sampling
Major Unknowns
• Which transmission route is dominant: direct contact, indirect contact with contaminated objects (fomites), contact with droplets, inhalation of aerosols?
• How much virus is released in what size droplets/aerosols at different stages of infection?
• How are viruses inactivated in air and on surfaces?
Linsey Marr, Virginia Tech, July 2020 26 Liu et al., 2016, Indoor Air, https://onlinelibrary.wiley.com/doi/abs/10.1111/ina.12314
Acknowledgments Karen Kormuth Seema Lakdawala Weinan Leng Kaisen Lin AJ Prussin II Elankumaran Subbiah Eric Vejerano Peter Vikesland Haoran Wei Wan Yang
Linsey Marr, Virginia Tech, July 2020 27