Since Kilauea's massive eruption May 3, flaming rivers of molten rock have destroyed nearly 50 structures, according to the Hawaii Tribune Herald.

The Hawaii Tourism Authority recently distributed a guide, ”The Health Hazards of Volcanic Ash: A guide for the public,” prepared by the International Volcanic Health Hazard Network (IVHHN), Cities and Volcanoes Commission, GNS Science and the U.S. Geological Survey, to promote safety of those who experience volcanic ashfall.

Volcanic ash & other eruption fallout

Volcanic ash is composed of fine particles of fragmented volcanic rock (less than 2 mm diameter). Volcanic ash is often hot very close to the volcano but is cool when it falls at greater distances. It's formed during volcanic explosions, from avalanches of hot rock that flow down the side of volcanoes, or from red-hot liquid lava spray.

Ash varies in appearance depending upon the type of volcano and the form of the eruption. Thus, it can range in colour from light grey to black and can vary in size from being like grit to being as fine as talcum powder.

Related: Volcanic eruptions and your homeowners' insurance

Airborne ash blocks out sunlight, reducing visibility and sometimes causes complete darkness during day light.

Large ash deposits can incorporate into existing soils and become the future topsoil of a volcanic region. The fertility of the soils around many volcanoes is due to old ash deposits. This beneficial effect of volcanism outweighs, over time, the hazards from infrequent eruptions, so fertile volcanic areas are often densely populated

Damage caused by ash

Freshly fallen ash particles can have acid coatings which may cause irritation to the lungs and eyes. This acid coating is rapidly removed by rain, which may then pollute local water supplies. Acidic ash can also damage vegetation, leading to crop failure.

In most eruptions, volcanic ash causes relatively few health problems, but generates much anxiety. People can be more fearful of the health hazards of volcanic ash and gases than of the risk of dying from more major hazards, such as pyroclastic flows. However, ashfalls can affect very wide areas around volcanoes and may cause major disruption to normal living.

Above is a slideshow of the top health and property impacts of large ashfalls.

Related: Meet America's 18 'very high risk' volcanoes

Human health is always a major concern during volcano eruptions.

Common respiratory symptoms of volcanic ash exposure

• Nasal irritation and discharge (runny nose).
  Throat irritation and sore throat, sometimes accompanied by dry coughing.
• People with pre-existing chest complaints may develop severe bronchitic symptoms which last some days beyond exposure to ash (for example, hacking cough, production
  of sputum, wheezing, or shortness of breath).
• Airway irritation for people with asthma or bronchitis; common complaints of people with asthma include shortness of breath, wheezing and coughing.
• Breathing becomes uncomfortable.

In rare circumstances, long-term exposure to fine volcanic ash may lead to serious lung diseases. For these diseases to occur, the ash must be very fine, contain crystalline silica (for the disease silicosis to occur) and the people must be exposed to the ash in high concentrations over many years.

Exposure to crystalline silica in volcanic ash is typically of short duration (days to weeks), and studies suggest that the recommended exposure limits (similar in most countries) can be exceeded for short periods of time for the general population.

People suffering from asthma or other lung problems such as bronchitis and emphysema, and severe heart problems are most at risk.

Factors that impact respiratory symptoms

The development of respiratory symptoms from the inhalation of volcanic ash depends on a number of factors. These include the:

• Concentration of particles in the air.
• Proportion of fine particles in the ash.
• Frequency and duration of exposure.
• Presence of crystalline silica and volcanic gases or aerosols mixed with the ash.
• Meteorological conditions.
• Existing health conditions and the use of respiratory protective equipment will also influence the symptoms experienced.