Common chemical hazards and toxicity

With the progress of science, more and more chemical substances benefit people, but at the same time it poses a great threat to humans and the environment. Currently, there are more than 600,000 kinds of chemicals in the world, about 30,000 species have significant or potentially dangerous. These chemical dangerous goods are safe under certain external conditions. However, when they are affected by certain factors, serious conditions such as burning, explosion and poisoning may occur, which may cause harm to life and property. Therefore, people should understand these chemical dangerous goods more clearly and understand the nature, nature, and harmfulness of these chemicals before they can use effective scientific methods for effective management.

I. Chemical Dangerous Goods and Their Classification

Chemical dangerous goods are any chemicals that have the properties of burning, explosion, poisoning, corrosion, etc., and are liable to cause personal injury or property damage during production, storage, handling, transportation, etc. According to their dangerous characteristics, chemical dangerous goods are divided into 8 categories: Explosives, Compressed gases and liquefied gases, Flammable liquids, Flammable solids, Spontaneous combustion articles and flammable articles in contact with water, Oxidizing agents and organic peroxides, Toxic substances, Radioactivity Items, corrosion products.

Category I: Explosives

This type of chemical means that under the influence of the external environment ( such as heat, pressure, impact, etc. ) , it can produce violent chemical reactions, instantaneously generate a large amount of gas and heat, so that the surrounding pressure rises rapidly, an explosion occurs, causing damage to the surrounding environment article. It also includes items that have no overall explosion hazard, but have a risk of burning, projectiles, and minor explosions.

The second type: compressed gas and liquefied gas

This category of chemicals refers to compressed, liquefied or pressurized dissolved gases and should meet one of the following two conditions:

1 The critical temperature is lower than 50 °C , or at 50 °C , its vapor pressure is greater than 294kPa compressed or liquefied gas.

2 At a temperature of 21.1 °C , the absolute pressure of the gas is greater than 249 kPa ; or at 37.8 °C , the liquefied gas with Reid vapor pressure greater than 275 kPa and the gas dissolved and dissolved.

The third category: flammable liquids

This category of chemicals refers to flammable liquids, liquid mixtures or liquids containing solid matter, but does not include liquids that have been classified in other categories by their hazardous characteristics. Its closed cup flash point is equal to or lower than 61 °C .

Category IV: Flammable Solids, Spontaneous Combustion Articles, and Wet Combustible Articles

Flammable solids are those that have a low flash point, are sensitive to heat, impact, friction, are easily ignited by an external source of ignition, and burn rapidly, and may emit toxic fumes or toxic gases, but do not include items that have been included in explosives.

Self-igniting articles are articles that have a low self-ignition temperature, are prone to oxidation reactions in the air, emit heat, and burn themselves.

In case of wet flammable substances, it refers to violent chemical reactions that emit large amounts of flammable gases and heat when water or moisture is encountered. Some can burn or explode without an open flame.

Category 5: Oxidants and Organic Peroxides

Oxidants are substances that are in a highly oxidized state and have strong oxidizing properties and are easily decomposed and emit oxygen and heat. Including inorganic substances containing peroxy groups, which are not necessarily flammable per se, but can lead to the combustion of flammable substances, and can form an explosive mixture with a loose powdery combustible substance, and are sensitive to heat, vibration, or friction. Organic peroxides are organic compounds containing a peroxygen in their molecular composition. They are inherently flammable, explosive, easily decomposable, and extremely sensitive to heat, vibration, or friction.

Category 6: Drugs

This category of chemicals refers to a certain amount accumulated after entering the body, which can undergo biochemical or biophysical interactions with body fluids and organ tissues, disturbing or destroying normal physiological functions of the body, and causing certain organs and systems to temporarily or permanently Sexual pathological changes, even life-threatening items. Oral ingestion LD50: Solid LD50 < 500 mg/kg , liquid LD50 < 2000 mg/kg ; LD50 < 1000 mg/kg LD50 , dust; smoke and vapor inhalation LC50 < 10 mg/L after skin contact for 24 hours Solid and liquid.

Seventh Class: Radioactive Goods

This category refers to articles with a specific radioactivity greater than 7.4 × 104 Bq/kg .

Eighth class: Corrosion products

This category refers to solids or liquids that can burn human tissue and cause damage to metal and other items. Contact with the skin is visible necrosis within 4 hours, or at a temperature of 55 ℃, 20 uniform on the surface of the steel corrosion rate of more than 6.25mm / year solid or liquid. The toxicities of toxic chemicals are classified into five levels according to the international general classification method. The grading standard is based on the half lethal dose (LD50) of the test animals . See Table 7-1 .

Toxicity classification

The degree of harm caused by toxic chemicals is classified into four levels according to the “Classification of Occupational Exposure to Toxic Hazard Levels” : Extremely Hazardous, Highly Hazardous, Moderate Hazardous, and Lightly Hazardous. Acute toxicity indicators at all levels are based on LC50 and LD50 . See Table 7-2 .

Toxic dose level

In chemical accidents, the hazards of chemical toxicants to the body are generally expressed in terms of toxic doses and toxic dose levels.

1. toxic dose

It refers to the amount of poisons that can produce toxic effects on the body. Due to the different ways in which poisons enter the body, the poisoning dose is expressed differently.

(1) Toxic poisoning toxic vapor poisoning dose The toxic vapor poisoning dose generally depends on the concentration of toxic substances in the air and the exposure time of personnel in toxic air. Therefore, the poison dose (D) is expressed as the product of exposure concentration (C) and exposure time (T ) . This dose is also called concentrated hour product. Typically in micrograms · min / liter (μ g · min / L) , mg · min / liter (mg · min / L), mg · min / cubic meter (mg · min / m3) units.

Poison dose (D) = exposure concentration (C) × Exposure time (C)

(2) The toxic dose of poisoning caused by the absorption of liquid poisons through the skin generally depends on the amount of skin and the weight of the human body. The toxic dose was expressed as the weight of toxicant per kilogram of body weight. Usually in mg / kg (mg/kg) .

2. Poison dose level

It refers to the toxic dose corresponding to the level of injury. The difference in toxicant dose levels at different dose levels was compared with different toxicants and the toxicity of the toxicants was compared. The commonly used toxic dose levels are shown in Table 7-3 .

Second, the main characteristics of chemical accidents caused by chemical accidents

Dangerous chemicals, dangerous accidents, and even catastrophic accidents are related to their own characteristics.

1. Flammable and explosive

Inflammable and explosive chemicals can ignite and explode under the normal temperature and pressure, through the action of impact, friction, heat source, spark and other fire sources.

The ability to burn an explosion depends on the chemical composition of this material. The chemical composition determines the ignition point of the chemical, the flash point, the combustion range, the explosion limit, the burning rate, and the amount of heat generated.

In general, gases are more flammable and explosive than liquids and solids, and they burn faster. This is because the intermolecular force of the gas is small and it is easy to break bonds without dissolution, melting and decomposition.

The smaller the molecule, the lower the molecular weight of the material, the more active its chemical properties, and can easily cause combustion explosion. A gas composed of a simple composition is more flammable than a gas composed of a complex composition, and the saturating bonds of the valence bonds are flammable, such as fire and explosion hazard H2 > CO > CH4 .

The flammable gas must be mixed with the combustion-supporting gas before combustion. If the combustible gas leaks from the container and mixes with the air, it will form a mixture of air and combustible gas. When the mixed gas reaches a certain concentration range, it will be ignited in the event of a fire. The oxidizing agent that helps the combustion and the combustible gas that acts as the reducing agent are mixed to form an explosive mixture. The two conditions are mutually exclusive. The decomposition of explosive gases, such as ethylene, acetylene, ethylene oxide, etc., does not need to be mixed with the combustion-supporting gas, and it will itself explode. Some chemicals cannot be contacted with each other, or they will explode, such as nitric acid and benzene, potassium permanganate and glycerol.

Since static electricity is generated by the friction of any object, static electricity can be generated when a flammable or explosive chemical hazardous article is discharged from a damaged container or pipe port at a high speed. The more impurities in these gases or liquids, the faster the flow rate is. The more static charges there are, this is a very dangerous source of ignition. Dangerous goods with lower flash point are more flammable, such as yellow phosphorus when it meets air at room temperature. Chemicals that are wet and flammable will emit oxygen when exposed to moisture or water, such as calcium carbide and phosphorus pentoxide.

2. Diffusion

Chemical spills in chemical accidents can spread to the surroundings, lighter than air can diffuse in the air and diffuse, form a mixture with the air, and follow the wind, causing the spread of combustion, explosions, and poisons. Heavyer than air, it floats on the surface, ditch, corner, etc. It can accumulate for a long time and cause delayed fire, explosion and increased poison concentration, causing poisoning. The diffusivity of these gases is affected by the density of the gas itself, and the smaller the molecular weight, the faster the diffusion. If the molecular weight of hydrogen is the smallest, the diffusion rate is the fastest, and the time for reaching the explosion limit in air is the shortest. The diffusion rate of gas is inversely proportional to the square root of its molecular weight.

3. Sudden

The accidents caused by chemical substances are mostly sudden, and they cause harm in a very short time or in an instant. General fires have to go through several stages of fire, spread to violent combustion, and take several minutes to dozens of minutes. Once a chemical dangerous article catches fire, it begins to boom, spreads rapidly, and the burning and explosion alternate, together with toxic substances. Diffusion, rapid harm. Many chemical accidents are caused by the leakage of high-pressure gas from containers, pipes, towers, and tanks. Due to the nature of high-pressure gas, a large amount of gas is emitted in a short time, and a large area rapidly becomes a polluted area.

4. Poisoning

Toxic chemicals, whether fat-soluble or water-soluble, have the ability to enter the body and damage the body's normal function. When these chemicals enter the body in one or more ways for a certain amount, they will cause damage to the body structure. Destruction of normal physiological functions, causing poisoning.

Third, the main factors affecting the risk of chemical dangerous goods

The physical and chemical properties and status of chemical substances can indicate their physical and chemical hazards. For example, the density of gas and vapor can indicate whether the substance may flow along the ground or rise to the upper space. Heating, burning, polymerization, etc. can cause certain chemical substances to react chemically to cause explosion or generate toxic gas.

1. The relationship between physical properties and danger

1 Boiling point: At a pressure of 101.3 kPa (760 mmHg) atmospheric pressure, the temperature at which a substance changes from a liquid state to a gaseous state. The lower the boiling point of the substance, the faster the vaporization, and it is easy to quickly cause high concentrations of air pollution at the accident site.

2 Melting point: The substance 's dissolution temperature or temperature range at standard atmospheric pressure (101.3 kPa) . The melting point reflects the purity of the substance. It can be inferred that the substance is distributed in various environmental media ( water, soil, air ) . The melting point is related to the decontamination and contaminant treatment at the pollution site.

3 Relative density ( water is 1) : The ratio of the density of a substance to the density of water at 4 °C at ambient temperature (20 °C ) . It is an important parameter indicating whether the substance is floating on the water surface or sinking. When a fire with a relative density of less than 1 fires, the use of water to extinguish will be ineffective because the water will sink below the burning surface and the water can even spread the fire to distant locations due to its fluidity.

4 Vapor pressure: Abbreviation for saturated vapor pressure. This refers to the saturation pressure at which a substance equilibrates with its liquid or solid at a certain temperature. Vapor pressure is only a function of temperature. At a certain temperature, the saturated vapor pressure of each substance is a constant. The higher the temperature at the time of the accident, the higher the vapor pressure of the chemical substance and the corresponding increase in the air concentration.

5 Vapor Relative Density ( Air 1) : refers to the ratio of the vapor density of a chemical substance to the density of a reference substance ( air ) under given conditions . When the vapor relative density value is less than 1 , it means that the vapor is lighter than air and tends to rise in a relatively stable atmosphere. In a closed room, light gases tend to move toward the ceiling or escape from the open window. When the value is greater than 1 , it means that it is heavier than air, tends to concentrate near the ground after leaking, and can spread to a considerable distance at a lower position. If the gas is flammable and met the fire may cause distant fire back. If the released vapor is a combustible gas with a relative density of ≤ 0.9 , it may accumulate in the upper space of the building and cause an explosion. The common gas vapor density is shown in Table 7-5 .

Common gas vapor density

⑥ vapor / air mixture relative density (20 ℃, air is 1): refers to the solids above the liquid and the open air in contact with the presence of steam to air mixture with respect to the density of pure ambient air. When the relative density value ≥ 1.1 , the mixture may flow along the ground and may accumulate at low enthalpies. When the value is 0.9 to 1.1 , it can quickly mix with the surrounding air.

7 Flash point: The flash point represents the lowest temperature at which the flammable vapour released above the surface of a liquid is completely mixed with air and can be ignited by flame or spark at atmospheric pressure (101.3 kPa) . Flash point <21 ℃ the substance is a highly flammable material, 21 ℃ ≤ flash point ≤ 55 ℃ substance is flammable substances. The flash point is the basis for judging the flash of flammable liquid vapor due to an open flame. Flash point has two kinds of values, open and closed. Chemicals with a flash point of less than 21 °C leak out, easily forming an explosive mixture in the air, causing combustion and explosion.

8 Auto-ignition temperature: The lowest temperature at which a substance comes into contact with air, which may cause a fire or spontaneous ignition, and at which point the material can continue to burn without a source of ignition ( flame or spark ) . The autoignition temperature depends not only on the chemical nature of the material, but also on the size, shape, and properties of the material. The autoignition temperature is important for the choice of electrical equipment used in spaces where explosive vapor / air mixtures may be present .

9 Explosive Limit: A concentration range in which a combustible gas or mixture of steam and air can ignite or ignite an explosion. When the air contains combustible gases ( such as hydrogen, carbon monoxide, methane, etc. ) or vapors ( such as ethanol vapor, benzene vapor ) , in a certain concentration range, encountering a spark will cause the flame to spread and explode. The lowest concentration is called the lower limit, and the highest concentration is called the upper limit. If the concentration is lower or higher than this range, no explosion will occur. It is generally expressed as the volume percentage of combustible gas or vapor in the mixture. The combustible gas can be divided into two levels according to the lower explosion concentration, as shown in Table 7-6 .

 

10 Critical temperature and critical pressure: Some gases can become liquid under warming and pressure, and they are pressed into high-pressure cylinders or storage tanks. The maximum temperature at which the gas can be liquefied is called the critical temperature, and the minimum pressure required for liquefaction is called the critical pressure.

2. Other physical and chemical dangers

Electric conductivity is less than 104pS/m liquid in the flow, stirring can generate static electricity, causing fire and explosion, such as pumping, stirring, filtration and so on. This situation is more likely to occur if the liquid contains liquids, gases or solid particles ( mixtures, suspensions ) . Some chemical combustible materials, powder or fine particles ( diameter less than 0.5mm) , when fully mixed with air, may be ignited by ignition, in the enclosed space, the explosion may be very violent. Some chemicals generate peroxides during storage. Evaporated or heated residues may spontaneously explode. Such as ether compounds.

Polymerization is the chemical reaction of a substance's molecules in conjunction with macromolecules. Polymerization usually releases heat, which may cause pressure build-up, posing a fire or explosion hazard. Heating of some chemicals may cause violent burning or explosion. If it is heated or locally heated it reacts. This will lead to burning and may result in violent explosions in enclosed spaces. Some chemical substances, when mixed or burned with other substances, produce toxic gases that are released into space. For example, the combustion of almost all organic substances produces CO toxic gases. Another example: There are still some gases that are not toxic, but they are filled in large amounts in closed spaces, causing excessive saturation in the air and reducing the oxygen content and causing people to suffocate.

Strong acids and alkalis often react violently with other substances and cause corrosion.

3. Poisoning risk

In a sudden chemical accident, toxic chemicals can cause poisoning, and the danger is greatly increased. Poisoning can be classified according to the toxic effects of chemical substances:

Stimulant poisoning, such as: ammonia, chlorine, phosgene, sulfur dioxide, dimethyl sulfate, hydrogen fluoride, formaldehyde, chloroprene and so on.

Suffocating toxic poisons, such as: carbon monoxide, hydrogen sulfide, acrylonitrile, etc.

Anesthetic toxicity, mainly refers to some fat-soluble substances, such as: alcohols, esters, chlorohydrocarbons, aromatic hydrocarbons and so on. Anesthetic effects on nerve cells.

Methrohemoglobinemia causes increased methemoglobin levels and hypoxia in cells, such as aniline and nitro compounds.

Neurotoxicity can cause poisoning effect on the nervous system, such as organophosphorus, carbamate esters and the like pesticide, methyl bromide, phosphorus oxychloride, and phosphine.

Corrosive, cause skin burns by chemical substances with strong acid and alkali properties, or cause toxic poisoning by burns.

According to the state of the existence of chemical dangerous substances can be divided into gaseous, liquid, solid chemical poisons. Gaseous chemicals: The boiling point is low, and it is aerosol after release. The main types of chemicals are:

Chlorine, sulfur dioxide, ammonia, carbon monoxide, phosgene, vinyl chloride, nitrogen dioxide, etc.

Liquid chemical poisons: droplets after release, the main chemical substances are: chloropicrin, dichlorvos, benzene, allyl chloride, acetone, ether and so on.

Solid-state chemical poisons: Solid or powder after release. These chemicals are mainly potassium dichromate.

Generally speaking, the volatility and diffusivity of gaseous and liquid substances are larger than that of solids, which may cause large and extensive damages. The danger is greater than that of solids. However, some solid-state chemicals, when they interact with water or other substances, produce more toxic gases and liquids.

The toxicity of toxic chemicals is related to the chemical composition and structure of the substance. For example, compounds containing cyano, arsenic, mercury, and selenium are more toxic. The more volatile the poison is, it is easily absorbed by the respiratory tract and the more toxic it is. The greater the toxicity of the poison, the stronger the toxicity.

The information in this article comes from the Internet and was reorganized and edited by China Rescue Equipment Network.