Humans and horses can be susceptible to heat stress, and several factors can increase their vulnerability. Considering both rider and horse unique individuals might give you better insight to answer your question, “Is it too hot to have lessons?” 

I am hopeful that most horse professionals have seen the charts floating around showing the Heat Index vs Horses. And I think this is a fabulous place to start but I encourage you to consider the individual specifics on both the rider and the horse. 

For your riders

Several factors can make individuals more sensitive to heat leading to an increased risk of heat-related illnesses. Things to consider regarding your riders.

• Cardiovascular Conditions such as coronary artery disease, congestive heart failure, and hypertension can impair the body’s ability to regulate temperature. While Peripheral Artery Disease can reduce blood flow can impair heat dissipation.
• Endocrine Disorders: Overactivity of the thyroid gland increases metabolism, leading to higher body heat production. And diabetes can lead to poor blood circulation and nerve damage (neuropathy) can reduce the ability to sense and respond to heat.
• Neurological Conditions: Heat can exacerbate symptoms such as fatigue and weakness due to poor nerve conduction for people with Multiple Sclerosis (MS) and conditions like autonomic neuropathy can impair sweating and temperature regulation.
• Dermatological Conditions: Some genetic disorders that affect sweat gland development can reduce the ability to sweat and cool the body and severe burns or skin conditions can do the same.
• Chronic Respiratory Diseases: COPD and Asthma can cause exacerbated breathing due to heat and humidity. 
• Impaired kidney function can affect fluid balance and heat regulation.
• Chronic Rheumatologic and Autoimmune Conditions
• Lupus can cause photosensitivity and increase heat sensitivity.
• Excess body fat can insulate the body and reduce heat dissipation.
• Elderly and young children: Age-related changes in thermoregulation, reduced ability to sweat, and higher likelihood of chronic conditions.
• Mental Health Conditions such as Schizophrenia can impair the ability to regulate temperature 
• Anemia: Reduced oxygen-carrying capacity can affect energy levels and heat tolerance.

In addition, many medications can also impair a rider’s ability to tolerate heat such as:

• Diuretics: Increase urination and can lead to dehydration.
• Antihistamines: Can inhibit sweating.
• Antidepressants and Antipsychotics: Some can affect thermoregulation.
• Beta-blockers: Can reduce heart rate and the ability to dissipate heat.

Also, be sure to consider your riders communication skills and if they will be able to communicate to you if they are too hot!

For your horses

While the temperature may be ok for you horse when using the published charts that are circulating, but I encourage you to consider additional factors:

• Sweat production: Horses rely heavily on sweating to cool down. Some horses have a medical condition that reduces or eliminates their ability to sweat, while other horses can excessively sweat. Both of which need special care on hot days. 
• Respiratory Heat Loss: Horses do not rely significantly on panting to dissipate heat. This limits their ability to cool down through respiratory heat loss. Does this horse pant often? Is it too humid for their sweat to help cool them?
• Coat Characteristics: Horses with thick or long coats, especially in breeds adapted to colder climates, can struggle more in hot weather as their coat insulates and retains heat. In addition, dark-colored horses absorb more heat from the sun compared to light-colored horses so if you’re working in the sun this can be a big deal. 
• Your horses hydration status, fitness level and type of exercise that will be done is really important to consider. 
• Health Horses with health issues such as respiratory, cardiovascular or metabolic problems (like humans) are more vulnerable to heat stress. 
• Age: Very young, old horses also tend to have less efficient thermoregulatory systems.
• Consider acclimations – have temperatures been mild with a sudden spike? If so, the horses (and likely humans) have not acclimated to the hotter weather yet. 

So, if you have thought a little more about both your riders and your horses regarding the heat and you decided lessons are a go, what can you do to improve conditions for all?

• Shade
• Reduced duration of the lesson or move it early or late in the day
• Ventilation
• Have riders dress in light colored breathable clothing and consider if your horse needs a summer clip.
• Encourage hydration – have water in the arena for horses and riders! Take breaks and encourage drinking!
• Change you lesson plan – adjust to a lighter lesson with less physical demands
• Cool your horse and rider – The data is out there showing that the fastest way to help cool a horse down is by applying moderate temperature water. For your rider, cooling rags are a great option! Also as soon as it is safe, have your rider remove their helmet. 
• Opt for a ground lessons with a focus on horse knowledge, understanding and partnership!

After all, we all know…. You can lead a horse to water, but you can’t make them drink!