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Glossary · Transmission

Aerosol

Fine particles suspended in the air, capable of carrying infectious agents. Distinct from droplets by size (typically < 5 µm) and their ability to remain airborne for extended periods.

Also called : aerosols, airborne particles, droplet nuclei Transmission

An aerosol, in public health terms, refers to very fine particles suspended in the air that are capable of carrying microorganisms (viruses, bacteria, fungi). Aerosols have historically been distinguished from respiratory droplets by size — smaller, lighter, they remain airborne for longer and travel greater distances. Inhalation of aerosols from rodent droppings is how hantavirus is transmitted to humans.

Definition and thresholds

The historical 5 µm threshold

WHO long used a threshold of 5 micrometres (µm): particles larger than 5 µm are classified as respiratory droplets (short-range transmission, around 1 metre, rapid fall-out), and those smaller than 5 µm as aerosols or droplet nuclei (longer-range transmission, prolonged suspension). This threshold historically corresponds to the depth of penetration into the respiratory tree: particles < 5 µm can reach the pulmonary alveoli.

A dichotomy now softened

Since the COVID-19 pandemic, the scientific community has questioned this binary dichotomy. The size of expelled respiratory particles (coughing, sneezing, speaking, breathing) forms a continuum from 1 to 500 µm. WHO published a new conceptual framework in 2024 reflecting this evolution. In practice, the 5 µm threshold remains a useful pedagogical reference, but the sharp droplet/aerosol distinction is no longer considered strictly valid.

Transmission mechanisms

Aerosol transmission

Agents transmitted primarily by aerosols include tuberculosis, measles, chickenpox, and certain respiratory viruses during aerosol-generating procedures. For these agents, wearing a filtering FFP2 or N95 mask (filtration efficiency ≥ 94%) is required for respiratory protection — a standard surgical mask is insufficient.

Droplet transmission

Conversely, agents transmitted primarily by droplets (seasonal flu, meningococcus, mumps) fall to the ground within seconds of emission and do not reach the alveoli at distance. A surgical mask and a distance of 1 to 2 metres are generally enough to interrupt this transmission.

Hantavirus and aerosols

The main route of infection

Hantavirus disease is emblematic of an animal-to-human aerosol transmission. Reservoir rodents continuously shed the virus through their urine, faeces and saliva. When a human disturbs these materials (sweeping, vacuuming, handling bedding, cleaning an enclosed space), they suspend potentially infectious viral particles. Fine particles (< 5 µm) can reach the pulmonary alveoli, where the virus replicates and causes hantavirus pulmonary syndrome.

Human-to-human transmission of Andes virus

Human-to-human transmission of Andes virus, the only documented case among hantaviruses, appears to occur at short range during close and prolonged contact. Studies of the Epuyén outbreak suggest transmission via short droplets and possibly aerosols in immediate proximity, with no demonstration of wide airborne transmission over several metres as with influenza or COVID-19. This is one of the reasons WHO assesses the risk to the general population as low.

Practical consequences

For the general public in a potentially contaminated area

The CDC recommends, for cleaning any space that may be contaminated by rodent droppings: no dry sweeping, no vacuuming. The correct procedure is to spray a diluted bleach solution (1 part to 9 parts water), let it sit for 5 minutes, then wipe with disposable absorbent paper. Wearing an FFP2/N95 mask is recommended.

For healthcare workers

The management of a suspected or confirmed case of hantavirus disease, particularly Andes virus, requires wearing an FFP2/N95 mask, a waterproof gown, gloves and goggles or a face shield. This protection is particularly important during procedures with a risk of aerosol generation such as intubation, tracheal suctioning and respiratory physiotherapy.

Key figures

Standards & references

Frequently asked questions

How is hantavirus transmitted by aerosol?

The primary route of hantavirus infection is the inhalation of aerosols from the urine, faeces or saliva of carrier rodents. These aerosols form when contaminated materials are disturbed (dry sweeping, vacuuming, handling of bedding, opening a closed space colonised by rodents). Once inhaled, particles fine enough (< 5 µm) can reach the pulmonary alveoli, the site of viral replication.

Should we worry about airborne hantavirus transmission like influenza?

No. For human-to-human transmission of Andes virus (the only documented case), studies of the Epuyén outbreak suggest short-range transmission during close and prolonged contact. There is no wide airborne transmission over several meters as with influenza or COVID-19. The FFP2 masks worn during the MV Hondius disembarkation are a precautionary measure.

Why should you not sweep or vacuum a potentially contaminated space?

Dry sweeping and vacuuming suspend fine particles from rodent droppings, creating potentially infectious aerosols. The CDC explicitly recommends spraying a diluted bleach solution (1:9), letting it sit for 5 minutes, then wiping with disposable absorbent paper. This procedure avoids resuspending the particles.

Further reading