Syrian military forces dropped chlorine gas in rebel-held residential areas of Aleppo at least eight times late last year, killing four children and five other civilians in a protracted battle to retake the city, Human Rights Watch said in a report released Monday.


Roughly 200 people in eastern Aleppo were injured in the attacks, in which helicopters dropped chlorine cylinders in neighborhoods where government forces planned to advance, the report said.

“The pattern of the chlorine attacks shows that they were coordinated with the overall military strategy for retaking Aleppo, not the work of a few rogue elements,” Ole Solvang, deputy emergencies director at Human Rights Watch, said in the report.

It said the chlorine attacks started on Nov. 18 and continued until Dec. 9, days before the fight for Aleppo ended with a cease-fire and wide-scale evacuations of fighters and civilians.

If verified, the attacks would be a violation of the 1993 Chemical Weapons Convention, an international treaty Syria signed in 2013 that bans the use of chemical weapons.

Documenting the attacks was difficult because government forces confiscated phones and laptops and hospital workers had to leave records behind during the evacuation, the report said. Its conclusions are based on video footage, photographs and interviews with nearly two dozen witnesses, including medical workers, first-responders and residents.

Witnesses said that victims suffered symptoms consistent with chlorine attacks, including trouble breathing, nausea, fainting and foaming at the mouth. Video clips published with the report show plumes of green gas billowing from the city and in a neighborhood, as well as women and children being treated with oxygen masks.

Human Rights Watch called for the United Nations Security Council to impose sanctions on senior Syrian government leaders.

“Allowing the Syrian government to flaunt this prohibition with impunity runs the risk of implicitly condoning Syrian chemical attacks and undermining one of the most agreed-upon weapon bans in the world, potentially lowering the threshold for other countries to do the same,” Solvang said in the report.

The government has routinely denied using chemical weapons in the civil war.

Last year, a U.N. team investigating chemical weapons concluded that Syrian government forces used chlorine gas three times in 2014 and 2015.

In 2013, Human Rights Watch accused Syrian government forces of using sarin gas in attacks on two Damascus suburbs, killing hundreds of civilians.

Syrian helicopters used in past chlorine attacks operated from the Hmeymim air base, which is controlled by Russia, a staunch ally of the Syrian government that has aided its war effort.

Russia — which has also signed the treaty — should have prevented chemical weapons from being used in their joint military offensive, the latest report said.

Chlorine gas poisoning is illness resulting from the effects of exposure to chlorine beyond the threshold limit value.

The signs of acute chlorine gas poisoning are primarily respiratory, and include difficulty breathing and cough; listening to the lungs will generally reveal crackles. There will generally be sneezing, nose irritation, and throat irritation. There may also be skin irritation or chemical burns and eye irritation or conjunctivitis. A person with chlorine gas poisoning may also have nausea, vomiting, or a headache.

Chronic exposure to relatively low levels of chlorine gas may cause pulmonary problems like acute wheezing attacks, chronic cough with phlegm, and asthma.

Occupational exposures constitute the highest risk of toxicity and common domestic exposures result from the mixing of chlorine bleach with acidic washing agents such as acetic, nitric and phosphoric acid. They also occur as a result of the chlorination of table water. Other exposure risks occur during industrial or transportation accidents. Wartime exposure is rare.

Humans can smell chlorine gas at ranges from 0.1–0.3 ppm. According to a review from 2010: “At 1–3 ppm, there is mild mucus membrane irritation that can usually be tolerated for about an hour. At 5–15 ppm, there is moderate mucus membrane irritation. At 30 ppm and beyond, there is immediate chest pain, shortness of breath, and cough. At approximately 40–60 ppm, a toxic pneumonitis and/or acute pulmonary edema can develop…. Concentrations of about 400 ppm and beyond are generally fatal over 30 minutes, and at 1,000 ppm and above, fatality ensues within only a few minutes.”

The concentration of the inhaled gas and duration of exposure and water contents of the tissues exposed are the key determinants of toxicity; moist tissues like the eyes, throat, and lungs are the most susceptible to damage.

Once inhaled, chlorine gas diffuses into the epithelial lining fluid (ELF) of the respiratory epithelium and may directly interact with small molecules, proteins and lipids there and damage them, or may hydrolyze to hypochlorous acid and hydrochloric acid which in turn generate chloride ions and reactive oxygen species; the dominant theory is that most damage is via the acids.

Test performed to confirm chlorine gas poisoning and monitor patients for supportive care include pulse oximetry, testing serum electrolyte, blood urea nitrogen (BUN), and creatinine levels, measuring arterial blood gases, chest radiography, electrocardiogram (ECG), pulmonary function testing, and laryngoscopy or bronchoscopy.

There is no antidote for chlorine poisoning; management is supportive after evacuating people from the site of exposure and flushing exposed tissues. For lung damage caused by inhalation oxygen and bronchodilators may be administered.

There is no way to predict outcomes. Most people with mild to moderate exposure generally recover fully in three to five days, but some develop chronic problems such as reactive airway disease. Smoking or pre-existing lung conditions like asthma appear to increase the risk of long term complications.